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/user_name_change_requests_controller.rb

@@ -20,7 +20,7 @@ class UserNameChangeRequestsController < ApplicationController
   end
 
   def index
-    @change_requests = UserNameChangeRequest.visible.order("id desc").paginate(params[:page], :limit => params[:limit])
+    @change_requests = UserNameChangeRequest.visible(CurrentUser.user).order("id desc").paginate(params[:page], :limit => params[:limit])
     respond_with(@change_requests)
   end