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/lib/guardian_extensions.rb

@@ -153,10 +153,10 @@ module Chat::GuardianExtensions
     return false if !can_modify_channel_message?(message.chat_channel)
 
     if message.user_id == current_user.id
-      case chatable.class.name
-      when "Category"
+      case chatable
+      when Category
         return can_see_category?(chatable)
-      when "DirectMessageChannel"
+      when DirectMessage
         return true
       end
     end