FIX: We should use category_id instead of category_name to perform

operations, now that the subcategory names are not unique.

app/serializers/topic_tracking_state_serializer.rb

@@ -1,3 +1,8 @@
 class TopicTrackingStateSerializer < ApplicationSerializer
-  attributes :topic_id, :highest_post_number, :last_read_post_number, :created_at, :category_name, :notification_level
+  attributes :topic_id,
+             :highest_post_number,
+             :last_read_post_number,
+             :created_at,
+             :category_id,
+             :notification_level
 end