Implement the bors auto r?eassignment on r+ policy

[Centril]

See:

• https://github.com/rust-lang/rust/issues/59489
• https://github.com/rust-lang/rust/issues/59489#issuecomment-478589583

[Centril]

cc @kennytm

[bryanburgers]

Do you expect homu to set the assignee directly? Or do you expect homu to leave a r? at-person comment?

[Centril]

The former would generate least noise in terms of pings so it seems be preferable but the latter is also workable.

[varkor]

For anyone looking to work on this, you'll want to assign the new reviewer here: https://github.com/rust-lang/homu/blob/91da2d8e74d2140222f3e6a48f8442be9ef34331/homu/main.py#L567-L578 I'm not sure if bors already has a way to assign a reviewer, though, as @rust-highfive is usually responsible for that. Alternatively, it might be easier for @rust-highfive to do.