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/presenters/user_presenter.rb

@@ -152,6 +152,8 @@ class UserPresenter
   end
 
   def previous_names(template)
-    user.user_name_change_requests.map { |req| template.link_to req.original_name, req }.join(", ").html_safe
+    user.user_name_change_requests.visible(CurrentUser.user).map do |req|
+      template.link_to req.original_name, req }.join(", ").html_safe
+    end
   end
 end