a8n: Rename Campaign.changesetCreationStatus to .status

Created by: mrnugget

As requested here: https://github.com/sourcegraph/sourcegraph/pull/7346#discussion_r365306633

Also makes a lot of sense in the context of updating a Campaign.

I also made it non-nullable.

@eseliger @felixfbecker can you check whether my TypeScript changes are okay?