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/serializers/direct_message_serializer.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class DirectMessageSerializer < ApplicationSerializer
+  has_many :users, serializer: UserWithCustomFieldsAndStatusSerializer, embed: :objects
+
+  def users
+    users = object.direct_message_users.map(&:user).map { |u| u || DeletedChatUser.new }
+
+    return users - [scope.user] if users.count > 1
+    users
+  end
+end