handle manual deletion of gist

[salomegeo]

User can manually delete a gist, by that time they might have exchanged this gist id to their friends.

Proposal:
If we ever see "FRIEND"'s signaling or heartbeat gist was deleted( we assume 404 means it was deleted), we resend invitation and change friend's status to invited.

If we ever receive invitation from a "FRIEND", automatically accept invitation.

In case heartbeat is deleted, user would have to log out and log back in to restore it.