Add "forum" namespacing to previously renamed core keys

- Does not affect "core.deleted_user" global string.
- Corresponding YAML will be sent later w/ more extracted strings.

js/forum/src/components/DiscussionRenamedPost.js

@@ -14,7 +14,7 @@ export default class DiscussionRenamedPost extends EventPost {
   }
 
   descriptionKey() {
-    return 'core.post_stream_discussion_renamed_text';
+    return 'core.forum.post_stream_discussion_renamed_text';
   }
 
   descriptionData() {