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/user_name_change_request.rb

@@ -8,10 +8,10 @@ class UserNameChangeRequest < ApplicationRecord
 
   after_create :update_name!
 
-  def self.visible(viewer = CurrentUser.user)
-    if viewer.is_admin?
+  def self.visible(user)
+    if user.is_admin?
       all
-    elsif viewer.is_member?
+    elsif user.is_member?
       where(user: User.undeleted)
     else
       none