Create idea of read-only database
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. Closes gitlab-org/gitlab-ce#37534.
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