users: rework privacy mode into private favorites (fix #4257).

* Rename 'privacy mode' to 'private favorites'.
* Make the private favorites setting only hide favorites, not favgroups
  and not the user's uploads on their profile page.
* Make the favgroup is_public flag default to true instead of false and
  fix existing favgroups to be public if the user didn't have privacy mode
  enabled before.
* List _all_ public favgroups on the /favorite_groups index, not just
  favgroups belonging to the current user.
* Add a /users/<id>/favorite_groups endpoint.

app/presenters/user_presenter.rb

@@ -88,7 +88,7 @@ class UserPresenter
   end
 
   def favorite_group_count(template)
-    template.link_to(user.favorite_group_count, template.favorite_groups_path(:search => {:creator_id => user.id}))
+    template.link_to(user.favorite_group_count, template.user_favorite_groups_path(user.id))
   end
 
   def comment_count(template)