Add setting to enable/disable HTML emails

Closes #24880

See merge request !7749

Add issue search slash command

See merge request !7752

Replace issue access checks with use of IssuableFinder

Split from !2024 to partially solve https://gitlab.com/gitlab-org/gitlab-ce/issues/23867

## Which fixes are in this MR?

:warning: - Potentially untested  
:bomb: - No test coverage  
:traffic_light: - Test coverage of some sort exists (a test failed when error raised)  
:vertical_traffic_light: - Test coverage of return value (a test failed when nil used)  
:white_check_mark: - Permissions check tested

### Issue lookup with access check

Using `visible_to_user` likely makes these security issues too. See [Code smells](#code-smells).

- [x] :vertical_traffic_light: app/finders/notes_finder.rb:15 [`visible_to_user`]
- [x] :traffic_light: app/views/layouts/nav/_project.html.haml:73 [`visible_to_user`] [`.count`]
- [x] :white_check_mark: app/services/merge_requests/build_service.rb:84 [`issue.try(:confidential?)`]
- [x] :white_check_mark: lib/api/issues.rb:112 [`visible_to_user`]
  - CHANGELOG: Prevented API returning issues set to 'Only team members' to everyone
- [x] :white_check_mark: lib/api/helpers.rb:126 [`can?(current_user, :read_issue, issue)`] Maybe here too?
- [x] :white_check_mark: lib/gitlab/search_results.rb:53 [`visible_to_user`]

### Previous discussions
- [ ] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#b2ff264eddf9819d7693c14ae213d941494fe2b3_128_126
- [ ] https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2024/diffs#7b6375270d22f880bdcb085e47b519b426a5c6c7_87_87

See merge request !2031

Fix label creation non members

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23416

See merge request !2006

This new global setting will allow admins to specify if HTML emails should be sent or not,
this is basically useful when system administrators want to save some disk space by avoiding
emails in HTML format and using only the Plain Text version.

Refresh project authorizations using a Redis lease

This MR changes `User#refresh_authorized_projects` so it uses a Redis lease instead of relying on serializable transactions. See the commit message(s) for more details.

See merge request !7733

Expose coverage on GET pipelines/:id

Closes gitlab-org/gitlab-ce#24801

See merge request !7670

Fix: Timeout creating and viewing merge request for binary file

See merge request !7713

The coverage wasn't exposed yet, now it is but only for detailed
requests to save queries on the database.

One of many requested in: gitlab-org/gitlab-ce#24768