PR Review limit for subrepos

[blyxyas]

Currently triagebot work show only works for reviewers of rust-lang/rust, we where talking about this in the Clippy meeting at the All Hands and seems that triagebot doesn't support it at all. (Neither work show nor work set-pr-limit 1)

The PR limit should be a global limit or it should be differenced between repos.

[Kobzol]

Yeah, currently it's only for rust-lang/rust. We should first test drive it there to see what works and what doesn't, and then we can think about how to extend it to other repositories. Due to some legacy stuff it's not trivial to do the PR tracking for multiple repos, implementation-wise, so we'll also need to do some refactoring there.