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/spec/lib/chat_review_queue_spec.rb

@@ -369,9 +369,7 @@ describe Chat::ChatReviewQueue do
     end
 
     context "when flagging a DM" do
-      fab!(:dm_channel) do
-        Fabricate(:direct_message_chat_channel, users: [message_poster, flagger])
-      end
+      fab!(:dm_channel) { Fabricate(:direct_message_channel, users: [message_poster, flagger]) }
 
       12.times do |i|
         fab!("dm_message_#{i + 1}") do