Make rebase-migration handle chains of migrations

[adamchainz]

From @tolomea again: If a feature branch has 2+ migrations in the same app, rebase-migration can't help with rebasing the migration. It should be possible to rebase an arbitrary linear chain of migrations by converting the existing code into higher-level abstractions.

[tolomea]

Additionally, currently when it encounters this situation it makes incorrect modifications, it would be preferable if it at least errored

[adamchainz]

PR's welcome