Fix campaign becoming out of date

Created by: eseliger

Before, we very manually fetched the campaign again, but I think the better approach here is to do it like we do with changesets: polling.

Since other users can also modify the campaign while you're looking at it, our "update only when the user triggered something" doesn't really work anyways.

Closes https://github.com/sourcegraph/sourcegraph/issues/13183