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.

config/routes.rb

@@ -268,6 +268,7 @@ Rails.application.routes.draw do
     end
   end
   resources :users do
+    resources :favorite_groups, controller: "favorite_groups", only: [:index], as: "favorite_groups"
     resource :password, :only => [:edit], :controller => "maintenance/user/passwords"
     resource :api_key, :only => [:show, :view, :update, :destroy], :controller => "maintenance/user/api_keys" do
       post :view