pius fails to sign User Attributes (attached photos)

[dkg]

similar to the note about e-mail-address-free User IDs in #109, pius should offer to certify user attributes (attached photos). If it does certify attached photos for a given OpenPGP cert, it should send those certifications to every e-mail address that it certifies as well on that certificate.

[jaymzh]

This is a complicated question and I've struggled with this quite a bit.

I don't want to blindly send this to all email addresses, I'd like to provide a choice. But I also need to actually display the image.

[jaymzh]

Actually I proposed a reasonable solution to this in #109 we should do that.