Fix the reviewed merge requests update on push

Connor, please see my email with subject "Another bug in my GitLab changes".
I ended up using the second described approach with hacking the 'opened' state.
There are two commits in the branch:
* 05deff01340bb8c036957d2aade5405a1f060a37 fixes the logic itself
* 5ed6e86e01b843bf8d9930771235ca3cdc97f168 adds a data migration to fix the old MRs in reviewed states