models: refactor search visibility methods.

Refactor how model visibility works in index actions:

* Call `visible` in the controller instead of in model `search`
  methods. This decouples model visibility from model searching.

* Explicitly pass CurrentUser when calling `visible`. This reduces
  hidden dependencies on the current user inside models.

* Standardize on calling the method `visible`. In some places it was
  called `permitted` instead.

* Add a `visible` base method to ApplicationModel.

app/controllers/favorite_groups_controller.rb

@@ -4,7 +4,7 @@ class FavoriteGroupsController < ApplicationController
 
   def index
     params[:search][:creator_id] ||= params[:user_id]
-    @favorite_groups = FavoriteGroup.paginated_search(params)
+    @favorite_groups = FavoriteGroup.visible(CurrentUser.user).paginated_search(params)
     @favorite_groups = @favorite_groups.includes(:creator) if request.format.html?
 
     respond_with(@favorite_groups)