DEV: Rename direct message related models

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.

plugins/chat/app/models/direct_message_user.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class DirectMessageUser < ActiveRecord::Base
-  belongs_to :direct_message_channel
+  belongs_to :direct_message, foreign_key: :direct_message_channel_id
   belongs_to :user
 end