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/test/javascripts/acceptance/user-card-chat-test.js

@@ -51,7 +51,7 @@ acceptance("Discourse Chat - User card test", function (needs) {
             ],
           },
           chatable_id: 16,
-          chatable_type: "DirectMessageChannel",
+          chatable_type: "DirectMessage",
           chatable_url: null,
           id: 75,
           title: "@hawk",