Add the option to approve a PR once PR CI passes

[Kobzol]

Suggested here.

[Sakib25800]

Going to take a crack at this

[Kobzol]

It's kind of unclear whether we want this or #317.

Also, one thing we found out is that the current check runs that we attach to PRs actually mark the PR as having "PR CI failed" if the check run fails. That's not really ideal, as it would prevent us from recognizing when PR CI is green.

[Sakib25800]

@Kobzol I wanted to pick one of these up, but not sure which. It looks like that discussion leaned towards #317 instead of this?

[Kobzol]

I'm not sure, tbh, I think that this still requires more discussion before going to implementation. There are various edge cases that could make this more harmful than useful if we do it automatically and not as an opt-in. We should resurrect the discussion in https://rust-lang.zulipchat.com/#narrow/channel/496228-t-infra.2Fbors/topic/bors.20r.2B.20after.20pr.20ci.20passes/with/510466306.

[Kobzol]

We brainstormed a tentative design for this in #t-infra > bors r+ after pr ci passes @ 💬.