Support for account migrations from Wildebeest to Mastodon

[japzone1]

As mentioned in #238, being able to migrate accounts and data is critical to confidence in a federated platform. Please implement this as it'll give potential users more confidence to try Wildebeest if they know they won't get stuck there if it doesn't workout.

[sterlingwes]

Looks like this is making progress based on recent commits.🎉🔥

Would be amazing if we could get a simple roadmap / checklist from maintainers on what's left so we know which commits to wait for 👀