Add rollup logic

[Sakib25800]

This PR adds the rollup logic. You can select certain PRs and then create a rollup.

I did not add the "create a similar PR" text in the rollup PR description as I'm not sure it's used - I can add that in another PR if needed.

The rollup logic works very similarly to homu's.

[fmease]

I did not add the "create a similar PR" text in the rollup PR description as I'm not sure it's used

I've used it very often. It's super helpful for creating a new rollup based on a failed one where you've kicked out a single PR. I'm pretty sure everyone who does rollups uses it all the time. You don't want to manually reassemble the failed rollup -1 bad PR (+N new ones) when there are like 30+ PRs in the queue.

Now, I've heard that this bors impl will have other tricks up its sleeve eventually esp. for creating multiple rollups at once / staggered, so maybe such a link will indeed be made obsolete via some other functionality / mechanism.

[Kobzol]

I did not add the "create a similar PR" text in the rollup PR description as I'm not sure it's used - I can add that in another PR if needed.

I think that button is indeed quite useful. However, we don't need to implement it in this PR, it can be follow-up work.

[Kobzol]

Thank you! The PR needs a rebase, and maybe a squash (17 commits is a bit too much), but other than that I think I'm ok with merging! We can take a look at parallelizing the PR fetch later. Awesome job! :heart: