Remote apps reopen reliably after a dropped connection

When a card's workspace is on another machine, Obeya reaches the app there through an ssh forward. If that connection dropped, opening the app right afterwards could briefly lead to a dead address and an empty page, because Obeya noticed the drop only once the ssh process had ended. Obeya now checks that the forward still answers before handing out the address, and rebuilds it on the same port if not, so an open tab keeps working.