FIX: whisper no longer experimental

- Regular users are not notified of whispers
- Regular users no longer have "stuck" topics in unread
- Additional tracking for staff highest post number
- Remove a bunch of unused columns in topics table

spec/fabricators/topic_user_fabricator.rb

@@ -0,0 +1,4 @@
+Fabricator(:topic_user) do
+  user
+  topic
+end