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/comment.rb

@@ -142,6 +142,7 @@ class Comment < ApplicationRecord
     select { |comment| comment.visibility(user) == :hidden }
   end
 
+  # XXX rename
   def self.visible(user)
     select { |comment| comment.visibility(user) == :visible }
   end