notes_actions.rb

In the Snippets::NotesController the noteable was resolved and
authorized through the :snippet_id, so by passing a :target_id for a
different snippet it was possible to create a note on a snippet
where the user would be unauthorized to do so otherwise.

This fixes the problem by ignoring the :target_id and :target_type from
the request, and using the same noteable for creation and authorization.