Default `user_can_reference?` to true when no attributes match

Now, a reference link with a `.gfm` class but without one of our
`data-*-id` attributes should be shown to the user rather than hidden.

lib/gitlab/markdown/redactor_filter.rb

@@ -27,7 +27,7 @@ module Gitlab
         elsif node.has_attribute?('data-user-id')
           user_can_reference_user?(node.attr('data-user-id'))
         else
-          false
+          true
         end
       end