Create idea of read-only database

What does this MR do?

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo secondary node). But in GitLab CE it also might be useful to have the "read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully lead to less errors introduced, doing write operations when there aren't allowed for read-only calls.

Are there points in the code the reviewer needs to double check?

See if app/controllers/sessions_controller.rb still works like before.

Why was this MR needed?

It backports gitlab-org/gitlab-ee!2954.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#37534.