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/forum_topics_controller.rb

@@ -23,7 +23,7 @@ class ForumTopicsController < ApplicationController
     params[:search][:order] ||= "sticky" if request.format.html?
     params[:limit] ||= 40
 
-    @forum_topics = ForumTopic.paginated_search(params)
+    @forum_topics = ForumTopic.visible(CurrentUser.user).paginated_search(params)
 
     if request.format.atom?
       @forum_topics = @forum_topics.includes(:creator, :original_post)