Change branches pages to load merged tag via AJAX

This allows branches page to load quickly while letting potentially long-lasting merged-into checks run later. This addresses issues #3924 and #12619.
55 jobs for async-load-merged-tags in 104 minutes and 29 seconds (queued for 25 seconds)
Status Job ID Name Coverage
  Prepare
passed #15032965
gitlab-org
knapsack

00:03:26

passed #15032966
gitlab-org
setup-test-env

00:15:01

 
  Test
passed #15033035
gitlab-org
bundler:check

00:06:44

passed #15033033
gitlab-org
docs:check:apilint

00:00:37

passed #15033034
gitlab-org
docs:check:links

00:00:55

passed #15033023
gitlab-org
license_finder

00:17:14

passed #15033036
gitlab-org
lint:javascript

00:02:05

passed #15033021
gitlab-org
rake brakeman

00:17:11

passed #15033020
gitlab-org
rake config_lint

00:10:18

passed #15033024
gitlab-org
rake downtime_check

00:17:58

passed #15033022
gitlab-org
rake flay

00:11:04

passed #15033031
gitlab-org
rake gitlab:assets:compile

00:17:22

passed #15033018
gitlab-org
rake haml_lint

00:06:53

passed #15033032
gitlab-org
rake karma

00:12:05

46.56%
passed #15033026
gitlab-org
rake mysql db:migrate:reset

00:04:20

passed #15033028
gitlab-org
rake mysql db:rollback

00:21:45

passed #15033030
gitlab-org
rake mysql db:seed_fu

00:09:24

passed #15033025
gitlab-org
rake pg db:migrate:reset

00:19:31

passed #15033027
gitlab-org
rake pg db:rollback

00:20:41

passed #15033029
gitlab-org
rake pg db:seed_fu

00:09:29

passed #15033019
gitlab-org
rake scss_lint

00:11:20

passed #15032980
gitlab-org
rspec pg 0 20

00:25:04

passed #15032982
gitlab-org
rspec pg 1 20

00:49:32

passed #15032994
gitlab-org
rspec pg 10 20

00:56:59

passed #15032995
gitlab-org
rspec pg 11 20

00:21:29

passed #15032996
gitlab-org
rspec pg 12 20

00:41:30

passed #15032997
gitlab-org
rspec pg 13 20

00:21:53

passed #15032998
gitlab-org
rspec pg 14 20

00:43:21

passed #15032999
gitlab-org
rspec pg 15 20

00:32:35

passed #15033000
gitlab-org
rspec pg 16 20

00:17:26

passed #15033001
gitlab-org
rspec pg 17 20

00:18:20

passed #15033002
gitlab-org
rspec pg 18 20

00:32:45

passed #15033003
gitlab-org
rspec pg 19 20

00:42:16

passed #15032984
gitlab-org
rspec pg 2 20

00:34:21

passed #15088488
gitlab-org
rspec pg 3 20

00:23:54

passed #15032988
gitlab-org
rspec pg 4 20

00:14:43

passed #15032989
gitlab-org
rspec pg 5 20

00:49:26

passed #15032990
gitlab-org
rspec pg 6 20

00:44:24

passed #15032991
gitlab-org
rspec pg 7 20

00:19:25

passed #15032992
gitlab-org
rspec pg 8 20

00:26:21

passed #15032993
gitlab-org
rspec pg 9 20

00:25:57

passed #15033017
gitlab-org
rubocop

00:11:29

passed #15033004
gitlab-org
spinach pg 0 10

00:20:28

passed #15033005
gitlab-org
spinach pg 1 10

00:13:26

passed #15033006
gitlab-org
spinach pg 2 10

00:16:00

passed #15033007
gitlab-org
spinach pg 3 10

00:15:31

passed #15033008
gitlab-org
spinach pg 4 10

00:11:20

passed #15033009
gitlab-org
spinach pg 5 10

00:18:31

passed #15033012
gitlab-org
spinach pg 6 10

00:14:41

passed #15033014
gitlab-org
spinach pg 7 10

00:13:20

passed #15033015
gitlab-org
spinach pg 8 10

00:13:36

passed #15033016
gitlab-org
spinach pg 9 10

00:14:34

failed #15032986
gitlab-org
rspec pg 3 20

00:15:47

 
  Post Test
passed #15033037
gitlab-org
coverage

00:08:25

92.47%
passed #15033039
gitlab-org
lint:javascript:report

00:03:00