In some instances the communities migration would be skipped but not
marked as `dirty`.
This commit addresses the issue by:
- Making sure that if dirty is set the migration is not skipped but
replayed
- If the version is on the communities migration and dirty is false, we
check for the presence of the communities table. If not present we
replay the communities migration.
- Make community_id field in user_messages nullable
It also removes all the `down` migration, as we can't use them
effectively, as explained in the README.md added.
Andrea Maria Piana