migrations: Enforce that repo.external is always set in DB

Created by: keegancsmith

This has been the case for nearly a year in Sourcegraph, but we still have the partial index from the days when it wasn't always set. This migration is a step forward to ensure we enforce this behaviour and will allow us to remove code which handles the unset case.

Note: On Sourcegraph.com we still had rows which didn't have the columns set due to not running the normal repo-updater sync. I manually investigated those rows and none seem "important". For the external data to be missing it implies no one has visited the repo for many months. This delete 500k rows from our repos table (still 4mil rows remaining).