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.
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.
