When we create a rollup, it would be great to remember in the DB which PRs are a part of it. So that we can provide some visualization of which PRs are currently in a rollup in the queue page.

This would likely be a new DB table (e.g. rollup_contents) that would associate a given rollup PR to another PR that is contained in it. Something like (repository TEXT NOT NULL, rollup_pr BIGINT NOT NULL, member_pr BIGINT NOT NULL), where (rollup_pr, member_pr) should be unique. Also some index would likely be helpful, so that we can quickly find all rollups in which a given PR is located.