Fixes issue with dropdown caret being incorrect color
Previously:

Now:

See merge request !3254
Follow the CSS Style Guide rules for HEX colors.
Use lowercase letters and shorthand values for HEX colors where possible, in accordance with the CSS Style Guide.
Discussed in #14299.
\cc @jschatz1 @rspeicher
See merge request !3238
Fix requirements.md link and improve PostgreSQL commands
- Point requirements.md link to the correct location
- Link to gitlab installation.md document instead of github
- Use psql command switch to run specified SQL commands for creation of user and database permissions.
- Instead of requiring a comment like `# Do not type the 'template1=#', this is part of the prompt` it may be easier to just use the command switch with `psql` to execute the SQL commands.
*forgot [ci skip] :(*
See merge request !3176
Support Wiki with UTF-8 page name
## What does this MR do?
Support Wiki with UTF-8 page name.
See https://github.com/gollum/gollum/pull/929
## Why was this MR needed?
Relax constraints for wiki slug in aac6598482.
It allows to create a wiki with UTF-8 name, but creating a wiki with UTF-8 name causes 500 error.
Creating a wiki with UTF-8 name once, then creating or updating wiki with ascii name also cause 500 error i.e. no one can create and update any wiki pages in the project.
## Workaround
1. Go to `https://DOMAIN/repo/wikis/git_access` -> this page display the link to git clone
2. Clone the wiki repo
3. Find the page with UTF-8 name
4. Rename or Delete these files
5. Commit and push
## What are the relevant issue numbers?
- #13979
- #13891
- #13698
- #13603
- #13317
- #12906
- #12825
- #10945
## Todo
- [x] Waiting for 'gollum-rugged_adapter' that support rugged v0.24.0 release
'gollum-rugged_adapter' gem doesn't allow to install rugged v0.24.0 (it's still beta version),
but 'gitlab_git' gem depends on rugged v0.24.0b13.
So it can't install both 'gollum-rugged_adapter' and 'gitlab_git' now.
See merge request !2999
Fixed overflow on users name in dropdowns
Closes#14289

See merge request !3235
Allow subscribing to labels
This implement #12743 and supersedes !2799 (thanks @timothyandrew!).
- Subscribe (and unsubscribe) for labels
- When an issue/merge request is created, notify all subscribers of all its labels
- When an issue/merge request is edited, notify all subscribers of all its newly-added labels
## Done
- [x] Verify that a user is signed in to subscribe/unsubscribe from a label.
- [x] Merge conflicts
- [x] Integration tests
- [x] `issuable.subscribed?` should check subscribers and participants
- [x] When an issuable is relabeled, notify subscribers of the *newly added* labels only
- [x] Screenshots:
### Labels page

### HTML email

### Plain text email

PS: I've set the milestone to 8.6 since it's getting late for 8.5...
See merge request !3115
1. Make the "subscribed" text in Issuable sidebar reflect the labels
subscription status
2. Current user mut be logged-in to toggle issue/MR/label subscription