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/controllers/favorite_groups_controller.rb

@@ -3,6 +3,7 @@ class FavoriteGroupsController < ApplicationController
   respond_to :html, :xml, :json, :js
 
   def index
+    params[:search][:creator_id] ||= params[:user_id]
     @favorite_groups = FavoriteGroup.paginated_search(params)
     respond_with(@favorite_groups)
   end