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/models/favorite_group.rb

@@ -27,7 +27,6 @@ class FavoriteGroup < ApplicationRecord
 
     def search(params)
       q = super
-      q = q.visible(CurrentUser.user)
       q = q.search_attributes(params, :name, :is_public, :post_ids, :creator)
 
       if params[:name_matches].present?