Refactor :comment_personal_snippet to :create_note

Follow-up to https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2794

We should probably keep this confidential until that MR is merged to master and issue is made public.

The :create_note permission is being checked on the Noteable when replying to email notifications. The previous MR adds the :create_note permission to ProjectSnippetPolicy.

This is a duplicate of an existing :comment_personal_snippet permission. We should refactor uses of :comment_personal_snippet to use the common :create_note permission instead.