Change GateAPI to allow a feature editor to set an assigned reviewer

[jrobbins]

We will allow a user who has permission to edit a feature to request a specific reviewer by setting a "No response" vote for that reviewer. That makes the requested reviewer show up in the list of votes, but it does not really oblige them to do a review.

[jrobbins]

This was one way to implement it, but instead of setting a "No response" vote, I think it would probably be better to have an explicit assigned_reviewer field in Gate.