Fix #5237: Deleted comments can be viewed by other users

* Fix it so non-moderators can't search deleted comments using the
  `updater`, `body`, `score`, `do_not_bump_post`, or `is_sticky` fields.
  Searching for these fields will exclude deleted comments.

* Fix it so non-moderators can search for their own deleted comments using the
  `creator` field, but not for deleted comments belonging to other users.

* Fix it so that if a regular user searches `commenter:<username>`, they
  can only see posts with undeleted comments by that user. If a moderator or
  the commenter themselves searches `commenter:<username>`, they can see all
  posts the user has commented on, including posts with deleted comments.

* Fix it so the comment count on user profiles only counts visible
  comments. Regular users can only see the number of undeleted comments
  a user has, while moderators and the commenter themselves can see the
  total number of comments.

Known issue:

* It's still possible to order deleted comments by score, which can let
  you infer the score of deleted comments.

app/models/application_record.rb

@@ -44,6 +44,10 @@ class ApplicationRecord < ActiveRecord::Base
         all
       end
 
+      def visible_for_search(attribute, current_user)
+        policy(current_user).visible_for_search(all, attribute)
+      end
+
       def policy(current_user)
         Pundit.policy(current_user, self)
       end