Refactor pd migrate code to abstract away boilerplate

[aubrika]

Currently the migration code features a lot of copy-pasted functions which are common to every migration, and for the purposes of ergonomics, readability and bug surface this is not ideal. This should be kept as fairly straightforward or simple refactor with a mind to facilitating future automated testing of migrations and enhancing testability.

Suggestions from backlog grooming:

• migrations should use the APP_VERSION, which would give us idempotency for free
• migrations should be refactored to take a StateWrite

[conorsch]

We're not prioritizing this (see p-low tag), but as we continue to write migrations, there will be ample opportunity to make the code more DRY.