Feature: migration support (embark/disembark in shuttle terminology)

[ringtailsoftware]

Feature request, support migration to/from Mastodon.

From what I've seen, migrating to Shuttlecraft just needs us to add knownAs to the account, then we will acquire new followers when Mastodon tells them to move.

To move from Shuttlecraft to Mastodon (once the user sets up knownAs on their target server), we just send all followers an ActivityPub Move message telling them where we've gone to.

https://github.com/mastodon/mastodon/pull/9629 https://www.w3.org/TR/activitystreams-vocabulary/#dfn-move