Follow up to 766bcbc6840c9d665055441bcd77616b3a96e10e
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 is a followup of the previous refactor where we created two new
models to handle all the dedicated logic that was present in the
`ChatChannel` model.
For the sake of consistency, `DMChannel` has been renamed to
`DirectMessageChannel` and the previous `DirectMessageChannel` model is
now named `DirectMessage`. This should help reasoning about direct
messages.
