Use GitHub review API to populate the reviewer(s)

Just an idea, what if @bors r+ and @bors r=somebody were removed in favor of @bors merge? Then:

• Bors would ensure there is at least one approving review in the GitHub review UI and no outstanding changes requested, consistent with the regular GH requirements (change requests can be dismissed if somebody else reapproves)
• Bors would use all approving reviews to populate the list of reviewers for the commit message (multiple reviewers are somewhat often done now with e.g. @bors r=my-username,somebody-else)
• Whoever commented @bors merge gets listed as the git commit committer

The advantages I see here are:

• The GitHub UI is more familiar to most people outside of rust-lang
• First class support for multiple reviewers, no need for whoever does r+ to find everyone who left a green check or said r=me
• Using the @bors merger as the committer means history will show "authored by Bob, committed by Alice" for some review context, rather than Bors always being the committer. This is some better use of Git flow and shows up nicely in GH's UI

Start by tracing how Bors handles @bors r+, @bors r=somebody, and @bors merge, then review the GitHub review API requirements described in the issue. Done means merge commands use approving reviews, reject outstanding change requests, populate all reviewers in the commit message, and record the commenter as committer.