Links in projects don't update after external_url is changed

Proposed solution

1. Move the CACHE_COMMONMARK_VERSION constant to the database, in application_settings.
   1. As described in https://gitlab.com/gitlab-org/gitlab-ce/issues/32789#note_30579916, we may want to keep a hard-coded constant and an administrator-changeable version.
2. Add a Rake task (or API call?) to bump this, which will therefore have the effect of forcing a re-render of all existing content.
3. Add this to the documentation about the external_url setting.

Summary

If the external_url setting is changed, links (such as issue branches from the issue), do not update to the new setting. The remote links for the projects do update just fine.

Steps to reproduce

1. Create an issue
2. Create an issue branch from the issue
3. Change external_url in /etc/gitlab/gitlab.rb
4. Call gitlab-ctl reconfigure
5. Go back to the issue and check the link for the issue branch. This is the previous setting for the external_url, not the current.

What is the current bug behavior?

The URL for the links is not updated when the external_url setting is changed.

What is the expected correct behavior?

The URL for the links should be updated when the external_url setting is changed.

Relevant logs and/or screenshots

Shows the external_url that is used in the remote links.

Shows that the same external_url is not used in the link (look at the bottom of the screen)

Output of checks

This is on an Omnibus installation of version 9.1.3.

Results of GitLab environment info

Expand for output related to GitLab environment info

'''
System information
System:         Ubuntu 16.04
Current User:   git
Using RVM:      no
Ruby Version:   2.3.3p222
Gem Version:    2.6.6
Bundler Version:1.13.7
Rake Version:   10.5.0
Redis Version:  3.2.5
Git Version:    2.11.1
Sidekiq Version:4.2.7
GitLab information
Version:        9.1.3
Revision:       2e4e522
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            http://hydrogen
HTTP Clone URL: http://hydrogen/some-group/some-project.git
SSH Clone URL:  git@hydrogen:some-group/some-project.git
Using LDAP:     no
Using Omniauth: no
GitLab Shell
Version:        5.0.2
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git '''

Results of GitLab application Check

Expand for output related to the GitLab application check

'''
Checking GitLab Shell ...
GitLab Shell version >= 5.0.2 ? ... OK (5.0.2)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
6/1 ... ok
2/2 ... repository is empty
6/5 ... ok
6/7 ... ok
6/8 ... ok
6/11 ... ok
4/12 ... ok
4/14 ... ok
4/15 ... ok
4/16 ... ok
4/17 ... ok
4/18 ... ok
6/19 ... ok
4/20 ... ok
4/21 ... ok
4/22 ... ok
2/23 ... ok
4/24 ... ok
4/25 ... ok
4/26 ... ok
6/27 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
Send ping to redis server: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Reply by email ...
Reply by email is disabled in config/gitlab.yml
Checking Reply by email ... Finished
Checking LDAP ...
LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab ...
Git configured with autocrlf=input? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory setup correctly? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
projects have namespace: ...
6/1 ... yes
2/2 ... yes
6/5 ... yes
6/7 ... yes
6/8 ... yes
6/11 ... yes
4/12 ... yes
4/14 ... yes
4/15 ... yes
4/16 ... yes
4/17 ... yes
4/18 ... yes
6/19 ... yes
4/20 ... yes
4/21 ... yes
4/22 ... yes
2/23 ... yes
4/24 ... yes
4/25 ... yes
4/26 ... yes
6/27 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.3.3)
Your git bin path is "/opt/gitlab/embedded/bin/git"
Git version >= 2.7.3 ? ... yes (2.11.1)
Active users: 15
Checking GitLab ... Finished
'''

Possible fixes

I'm not familiar enough with the code to offer a possible fix.