campaigns: patch changeset IDs when migrating

Created by: LawnGnome

In #13001, I added some "evil" to move campaigns back to their original IDs after everything was created. In my rush to do so, I committed one of the basic mistakes of evil-doing: not considering the broader effects of my evil; namely, that changesets have an effective (but unenforced) foreign key on the campaign in their JSON campaign_ids blobs.

Obviously, the solution here is more evil, in the form of an UPDATE query that patches the JSON en masse. You're welcome?

cc: @marekweb, as we'll need this in 3.19 as well.