Add migration to make model_group_id nullable

This is prerequisite to merging PR #226. The deploy order will have to be:

* Merge this PR, run the migration
* Merge #226, wait for it it get out in a snapshot, and then run that migration.

(I can pull the migration from #226 into a different PR if needed, there will likely be a merge conflict with this one anyways)

This migration is applying cleanly (and reverting cleanly) against a test db (thanks test-db!). I would like someone(s) to review it before I merge and run the migration however.