Help contributors rebase PR branches

[foolip]

I often ask people to rebase PRs to trigger new CI runs, and sometimes do it myself. Unfortunately, it's pretty easy to get it wrong and end up touching way too many files, causing way too many labels and reviewers being added. It's hard to recover from this state, both in terms of fixing the git history and getting rid of all the labels and reviewers. Cases I've noticed in the past day:

• https://github.com/web-platform-tests/wpt/pull/10377#issuecomment-544385357
• https://github.com/web-platform-tests/wpt/pull/13606#issuecomment-544255228
• https://github.com/web-platform-tests/wpt/pull/14221#issuecomment-443292390
• https://github.com/web-platform-tests/wpt/pull/14879#event-2212024002

Most PRs are created with the ability for other contributors to push to the branches. Therefore, it should be pretty straightforward to support a request like @wpt-pr-bot rebase, causing the branch to be rebased and force pushed.

I would certainly use that, but it wouldn't fix the whole problem because you can only use it if you know it exists.

[foolip]

A trivial case to show what this looks like in practice: https://github.com/web-platform-tests/wpt/pull/10571#issuecomment-544459487