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.
Closesgitlab-org/gitlab-ce#37534.
This commit adds a number of _html columns and, with the exception of Note,
starts updating them whenever the content of their partner fields changes.
Note has a collision with the note_html attr_accessor; that will be fixed later
A background worker for clearing these cache columns is also introduced - use
`rake cache:clear` to set it off. You can clear the database or Redis caches
separately by running `rake cache:clear:db` or `rake cache:clear:redis`,
respectively.
Francisco Javier López