forum: change style of new/approved/pending/rejected labels.

Change the new, approved, pending, and rejected labels in the forum to
use an outlined pill style (like user levels in user tooltips) instead
of a filled background style. This is less obtrusive and more readable
in dark mode.

app/helpers/forum_topics_helper.rb

@@ -18,6 +18,8 @@ module ForumTopicsHelper
       :approved
     elsif topic.category_name == "Tags" && topic.bulk_update_requests.present? && topic.bulk_update_requests.all?(&:is_rejected?)
       :rejected
+    else
+      nil
     end
   end