Put whole gitlab installation to read-only mode

If you don't want to mess with the user privileges (as done in this PR), one approach would be to change the gitlab-shell pre-receive hook.

If the pre-receive hook always "exit 1", that should disable any push to any repo.

Actually a better answer is to change this flag to true. This makes the entire instance read only and adds a nice header in the app.

I would be inclined to archive every project, which would prevent any changes (including to the wiki, issues, etc.).

Although, that doesn’t prevent new projects.