campaigns: Move background worker to repo-updater
Created by: ryanslade
This moves us towards making all code host requests from repo-updater allowing use to use a shared rate limiter.
We tried this in the past and it panicked due to a use of a global db connection. This doesn't appear to be an issue anymore and I've tested both manual campaigns and campaigns created using src-cli.
Closes: https://github.com/sourcegraph/sourcegraph/issues/7647