Prepare codebase for new campaigns workflow

Created by: mrnugget

This PR/branch contains the implementation of https://github.com/sourcegraph/sourcegraph/pull/10921 and serves as a merge-target for other PRs.

PRs targeting (or already merged into) this branch

Backend implementation ticket

We will only merge this into master when we're confident enough that it won't break the existing campaigns feature in a way that's unfixable before release.

Other PRs will target this branch and thus make the review process easier.