Create bot to assign new PR's to reviewers

[brson]

We want to start moving toward a module ownership system where certain groups are responsible for reviewing certain subsystems. To do this we need to make sure that PR's get routed to the appropriate person. Let's have a bot that just assigns new PR's to reviewers round-robin, and they are responsible for making sure the right reviewer sees it.

[brson]

Maybe use highfive.

[nrc]

highfive is now live - see #17380

I would like to improve it to assign a better reviewer and to r? all PRs without r? (not just from first time contributors)

[alexcrichton]

Would it be possible to assign PRs through the github boxes instead of just with a mention? Just a mention tends to get drowned in the rest of email sometimes.

[jdm]

https://developer.github.com/v3/issues/assignees/ doesn't list any ways to modify the assignee for some reason.

[jdm]

Nevermind, https://developer.github.com/v3/issues/#edit-an-issue is what you're looking for.

[brson]

Done.