Integrate highfive with GitHub's reviews.

[emilio]

See for example https://github.com/servo/servo/pull/13729, where the tags were manually added because highfive didn't detect that it was reviewed.

Don't know how GitHub's API exposes this though.

[jdm]

https://platform.github.community/t/how-can-we-approve-or-request-changes-on-pull-requests-using-api/350 says this can't be done through the API yet.

[jdm]

Ooh, nevermind. Looks like there was a webhook added recently: https://developer.github.com/v3/activity/events/types/#pullrequestreviewevent