Rework migrations

This merge will incorporate an effort to re-do the entire migration set so that a "baseline" database for GraceDB can be constructed from migrations. It includes some amount of initial data migrations. It's been developed on gracedb-dev2 and successfully incorporated in situ on gracedb-dev1.

See https://bugs.ligo.org/redmine/issues/5933 for significantly more information, along with a procedure for updating a GraceDB instance in situ.