Add reviewers functionality

Summary:

This diff adds reviewer functionality -- prepopulating the list with authorized reviewers and making sure that at least two are specified.

Future diffs can refine this.

Test Plan: 
Dogfood it! Try submitting a PR without reviewers and with some.

Reviewers:

  @barooo plz
  @gnarmis plz
  @kairuiwang plz
  @kochalex plz
  @lshepard plz
  @mpstreeter plz
  @nhatch plz
  @spo11 plz
  @khsia plz