Require profile to be marked as moved

[codl]

While thinking about UI for #9 I figured, hey, mastodon already has a field for "I've moved to X". Why not require that to be set, discouraging the laziest would-be spammers, and allowing us to put the target account in the sample message, and to go "hey did you forget something?" if the user wrote a custom message and didn't put the new account in it