FEATURE: correct muted category implementation

- Don't change tracking state on muted categories
- Exclude muted sub categories from parent

db/migrate/20140618001820_dont_auto_muto_topics.rb

@@ -0,0 +1,10 @@
+class DontAutoMutoTopics < ActiveRecord::Migration
+  def change
+    # muting all new topics was a mistake, revert it
+    execute 'DELETE FROM topic_users WHERE notification_level = 0 and notifications_reason_id =7 AND first_visited_at IS NULL'
+
+    execute 'UPDATE topic_users SET notification_level = 1,
+                                    notifications_reason_id = NULL
+            WHERE notification_level = 0 AND notifications_reason_id =7'
+  end
+end