Since the migration was added as a post migration, we'll try to add the constraint first, causing a NotNullViolation exception.
This only affects sites that were last deployed more than two days ago.
Follow up to 766bcbc684
Makes ChatMessage.last_editor_id and ChatMessageRevision.user_id
NOT NULL since they are always filled in now and the last commit
had a migration to backfill this data.
This commit adds last_editor_id to ChatMessage for parity with Post in
core, as well as adding user_id to the ChatMessageRevision record since
we need to know who is making edits and revisions to messages, in case
in future we want to allow more than just the current user to edit chat
messages. The backfill for data here simply uses the record's creating
user ID, but in future if we allow other people to edit the messages it
will use their ID.
