When a PR has a review requested, assign that user, as well

[reedstrm]

When a developer is added as a PR reviewer (i.e. when a review is requested), it would be useful to add them as an assignee, since then they will show up on the zendesk card for that PR. @meeragovindaraju has written a python script that does this via cli and the github API, which could probably be turned into a lambda method to register as a callback, but it seems to me that adding one-more-feature to staxly might the easier path to a solution.