change PR to "waiting on author" after "changes requested" review

[nikomatsakis]

It would be useful if rustbot changed the status of a PR to S-waiting-on-author after observing a "changes requested" review from the assigned reviewer.

Branched out from #1339

[oli-obk]

I don't think this is working anymore, or at least not for Draft PRs?

See rust-lang/rust#89745 for an example

[tgross35]

Seems like this still doesn't work so I opened #1857