repo-updater: Migrations improvements

Created by: tsenart

This change set makes each migration be retried on all errors until completion as well as making the enabled state migration delete and enable the disabled repos.

Together with the upcoming UI changes that prevent admins disabling repos, as well as the new enabled default of repos yielded by Sources, this ensures that they won't be touched a second time a migration is ran.

The alternative of keeping track of which migrations already ran proved to be more work than expected, so I'm putting this option forward for review.

Part of #2025 (closed)