Edit Contact: Hide email signatures for contacts that do not have a user/CMS account

When editing a contact, we have an option to add an email signature which is then used when sending emails from CiviCRM (via Contact > Actions > Email).

It looks something like this:

Expanding the signature block displays a WYSIWYG and text field to enter an email signature.

This small link is easy to overlook by experienced CiviCRM users, but pretty overwhelming to new CiviCRM users. Not only does expanding explode the UI in a pretty scary way, it's also in an area of CiviCRM that is very overwhelming (location type, On Hold, Mass Mail?, Primary?, Delete). Have you ever looked in the eyes of a user when they first see those options?

Anyway, small tiny improvement proposal: let's hide 'signature' for contacts-that-are-not-users, because it would have no purpose.

@justinfreeman Pinging you because I feel you may have thoughts on this.

Notes for me: signature_html in CRM/Contact/Form/Edit/Email.php, and add an 'if' in the tpl.