Assign reviewer as a 'reviewer' instead of 'assignee'

[frewsxcv]

https://github.com/blog/2291-introducing-review-requests

[jdm]

I'm only interested in making this happen if we can filter lists of PRs by the reviewer like https://github.com/servo/servo/pulls?q=is%3Aopen+is%3Apr+label%3AS-awaiting-review+assignee%3Ajdm .

[frewsxcv]

Agreed. I realized that wasn't possible until after I opened this.

[tetsuharuohzeki]

I'm only interested in making this happen if we can filter lists of PRs by the reviewer like

Now, we can filter it by https://github.com/blog/2306-filter-pull-request-reviews-and-review-requests

[jdm]

The other important consideration is whether the github API allows filtering by review requests, since http://www.joshmatthews.net/who-reviews-the-reviewers/ relies on that feature.