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/forum_post_vote.rb

@@ -8,7 +8,10 @@ class ForumPostVote < ApplicationRecord
   scope :down, -> {where(score: -1)}
   scope :by, ->(user_id) {where(creator_id: user_id)}
   scope :excluding_user, ->(user_id) {where("creator_id <> ?", user_id)}
-  scope :visible, -> { where(forum_post: ForumPost.permitted) }
+
+  def self.visible(user)
+    where(forum_post: ForumPost.visible(user))
+  end
 
   def self.forum_post_matches(params)
     return all if params.blank?