Implement the bors auto r?eassignment on r+ policy

[Centril]

See:

• Bors policy question: Auto-reassignment on r+ rust#59489
• Bors policy question: Auto-reassignment on r+ rust#59489 (comment)

[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:

homu/homu/main.py

Lines 567 to 578 in 91da2d8

	state.add_comment(comments.Approved(
	sha=state.head_sha,
	approver=approver,
	bot=my_username,
	))
	treeclosed = state.blocked_by_closed_tree()
	if treeclosed:
	state.add_comment(
	':evergreen_tree: The tree is currently closed for pull requests below priority {}, this pull request will be tested once the tree is reopened' # noqa
	.format(treeclosed)
	)
	state.change_labels(LabelEvent.APPROVED)

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.