fixes #463 - Snippet add/delete/edit permissions

Quick PR to address #463

Implements Controller checks of AuthUser permissions to add/edit/delete snippets.

Added 3 according I18n `en-message.php` strings.

I didn't alter `View's` logic *(hiding/disabling controls etc.)* since it's irrelevant for current backend template. Just a note for @project79 about this.

Edit: changed Views behaviour