EX: indicate when I have a dehydrated device

[uhoreg]

Under the assumption that dehydrated devices are hidden from the normal device view, EX should give some sort of indication that there is a dehydrated device, so that the server can't sneak in a phony device without the user noticing.

[BillCarsonFr]

Moved to future work pakage, rationale:

• EX only encrypts to dehydrated devices if they are correctly cross-signed, and EX locks you out if your session is not verified.
• Also, EX is already not showing any other devices (nor signaling new logins was it you)