pundit: convert user name change requests to pundit.

Fix discrepancy between index action and show action. The index
action allowed members to see name changes for undeleted users, but the
show action didn't.

app/models/user.rb

@@ -322,6 +322,10 @@ class User < ApplicationRecord
       User.level_string(value || level)
     end
 
+    def is_deleted?
+      name.match?(/\Auser_[0-9]+~*\z/)
+    end
+
     def is_anonymous?
       level == Levels::ANONYMOUS
     end