Add counters
 - Add counter to tip show page. User can see how many times their tip has been bookmarked.
 - Add counter to user show page. User can see the number of times their bookmarks have been
   bookmarked by other users.
 - Add erb for counters to Tip and User show pages.
 - Add bookmarked_tips method to user model.
 - Add bookmarked_tips to Users controller show action by creating an instance
   variable to encapsulate the method.