Add a Differential revision status tokenizer datasource

Summary:
Ref T2543. This adds a tokenizer, similar to the Maniphest tokenizer, so the hard-coded <select /> control in Differential ApplicationSearch can be replaced with a more flexible control that handles the addition of new statuses with more grace.

This only adds the new datasource.

Test Plan: Used /typeahead/class/ to preview the behavior of the new datasource.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18392