Add an API call to return subscribed threads for a user.

api/notifications_and_subscriptions.rb

@@ -2,6 +2,10 @@ get "#{APIPREFIX}/users/:user_id/notifications" do |user_id|
   user.notifications.map(&:to_hash).to_json
 end
 
+get "#{APIPREFIX}/users/:user_id/subscribed_threads" do |user_id|
+  handle_threads_query(user.subscribed_threads)
+end
+
 post "#{APIPREFIX}/users/:user_id/subscriptions" do |user_id|
   user.subscribe(source).to_hash.to_json
 end