oobmigrations: Consolidate out of band migration runners into a single package

Created by: efritz

This PR enables us to run the out of band migration runners in the migrator binary (or, at least moves the code to something importable). At this point, we are able to run the migrators as long as there's a running instance. Since this doesn't work for upgrade scenarios, we still have to isolate the runners so that they can run without access to frontend configuration, gitserver, etc. This will be done in future PRs over time.

Test plan

Tested by hand that migrators are still registered in the worker.