migrations: Make repo.stars not nullable

Created by: tsenart

Repo pagination by stars doesn't handle NULL values in stars column.

Fixes https://github.com/sourcegraph/customer/issues/579 and we'll cut a patch release with this. In prod, I'll run the first part of this migration ahead of time, as to not cause the infamous dirty database migration issue due to it taking too long.

UPDATE repo SET stars = 0 WHERE stars IS NULL;