Merge branch '20968-add-setting-to-check-unresolved-discussion' into 'master'

Add setting to only allow merge requests to be merged when all discussions are resolved

_Originally opened at !6385 by @rodolfoasantos._

- - -

## What does this MR do?
Based on #20968 this merge request adds setting only to allow merge requests to be merged when all discussions are resolved. 

## Are there points in the code the reviewer needs to double check?
Check if there are other points to add the resolved discussion setting

## Why was this MR needed?
Add the possibility to configure the project to only accept merge request when all discussions are resolved

## Screenshots

![only_allow_merge_if_all_discussions_are_resolved](/uploads/9388db9421da0214590ffab6fb29f985/only_allow_merge_if_all_discussions_are_resolved.png)

![only_allow_merge_if_all_discussions_are_resolved_msg](/uploads/b1bba0c72ad67d3a1b34718baa08526e/only_allow_merge_if_all_discussions_are_resolved_msg.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #20968

See merge request !7125
82 jobs for master in 72 minutes and 27 seconds (queued for 4 minutes and 56 seconds)
Status Job ID Name Coverage
  Prepare
passed #5878733
knapsack

00:01:09

 
  Test
passed #5879108
bundler:audit

00:07:55

passed #5879107
bundler:check

00:07:46

passed #5879090
license_finder

00:04:10

passed #5879105
lint-doc

00:01:09

passed #5879116
lint-javascript

00:06:22

passed #5879112
migration paths

00:12:42

passed #5879088
rake brakeman

00:02:29

passed #5879096
rake db:migrate:reset

00:02:09

passed #5879098
rake db:seed_fu

00:05:14

passed #5879093
rake downtime_check

00:04:04

passed #5879089
rake flay

00:04:09

passed #5879080
rake haml_lint

00:04:28

passed #5879081
rake scss_lint

00:01:37

passed #5878734
rspec 0 20

00:11:50

passed #5879017
rspec 0 20 ruby21

00:14:34

passed #5878735
rspec 1 20

00:26:05

passed #5879019
rspec 1 20 ruby21

00:28:32

passed #5878774
rspec 10 20

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passed #5879035
rspec 10 20 ruby21

00:26:30

passed #5878777
rspec 11 20

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rspec 11 20 ruby21

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rspec 12 20

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rspec 12 20 ruby21

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rspec 13 20

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rspec 13 20 ruby21

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passed #5878857
rspec 14 20

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rspec 14 20 ruby21

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rspec 15 20

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rspec 15 20 ruby21

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rspec 16 20

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rspec 16 20 ruby21

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rspec 17 20

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rspec 17 20 ruby21

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rspec 18 20

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rspec 18 20 ruby21

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passed #5878942
rspec 19 20

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rspec 19 20 ruby21

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rspec 2 20

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rspec 2 20 ruby21

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passed #5878749
rspec 3 20

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passed #5879027
rspec 3 20 ruby21

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passed #5878751
rspec 4 20

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passed #5879029
rspec 4 20 ruby21

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passed #5878757
rspec 5 20

00:45:07

passed #5879030
rspec 5 20 ruby21

00:38:36

passed #5878760
rspec 6 20

00:27:56

passed #5879031
rspec 6 20 ruby21

00:24:00

passed #5878771
rspec 7 20

00:41:09

passed #5879032
rspec 7 20 ruby21

00:44:41

passed #5878772
rspec 8 20

00:27:02

passed #5879033
rspec 8 20 ruby21

00:27:15

passed #5878773
rspec 9 20

00:17:14

passed #5879034
rspec 9 20 ruby21

00:21:25

passed #5879079
rubocop

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passed #5878950
spinach 0 10

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spinach 0 10 ruby21

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passed #5878958
spinach 1 10

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spinach 1 10 ruby21

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spinach 2 10

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spinach 2 10 ruby21

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spinach 3 10

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spinach 3 10 ruby21

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spinach 4 10

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spinach 4 10 ruby21

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spinach 5 10

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spinach 5 10 ruby21

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spinach 6 10

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spinach 6 10 ruby21

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passed #5878980
spinach 7 10

00:19:55

passed #5879071
spinach 7 10 ruby21

00:28:24

passed #5878999
spinach 8 10

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passed #5879073
spinach 8 10 ruby21

00:45:39

passed #5879015
spinach 9 10

00:50:21

passed #5879074
spinach 9 10 ruby21

00:59:41

passed #5879099
teaspoon

00:15:49

 
  Post Test
passed #5879118
coverage

00:04:01

89.52%
skipped #5879121
notify:slack
passed #5879119
trigger_docs

00:01:23

passed #5879117
update-knapsack

00:03:54

 
  Pages
passed #5879133
pages

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  Deploy
failed pages:deploy

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