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_vote.rb

@@ -10,7 +10,7 @@ class CommentVote < ApplicationRecord
   validate :validate_comment_can_be_down_voted
   validates_inclusion_of :score, :in => [-1, 1], :message => "must be 1 or -1"
 
-  def self.visible(user = CurrentUser.user)
+  def self.visible(user)
     if user.is_admin?
       all
     elsif user.is_member?