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/mod_action.rb

@@ -54,7 +54,7 @@ class ModAction < ApplicationRecord
     other: 2000
   }
 
-  def self.permitted(user)
+  def self.visible(user)
     if user.is_moderator?
       all
     else
@@ -65,7 +65,6 @@ class ModAction < ApplicationRecord
   def self.search(params)
     q = super
 
-    q = q.permitted(CurrentUser.user)
     q = q.search_attributes(params, :creator, :category, :description)
     q = q.text_attribute_matches(:description, params[:description_matches])