Commands: Undo-Migration

Resolves #631

Verification: Manual bashing. Will cover in functionals.
SOL. Don't delete your snapshot file. :smile: I don't think it's likely given that users are probably just trying to remove the last migration.

It would be possible to re-create it using the last migration. We could extend migrations further and, if there isn't a snapshot, we could use the one from the last migration (a la EF6)