Design UX for finding out whether contacts support image attachments

When the user upgrades to a version of Briar that supports image attachments, it may not be possible to send images to a given contact until the contact has also upgraded to a suitable version.

We need to let the user know whether it's possible to send images to each contact, and if not, what needs to change before it will be possible, and how the user will know when it's changed.

Subtask of #1241 (closed).