Support "none()" in Differential to find revisions with no (un-resigned) reviewers

Summary:
Ref T13289. In Maniphest, you can currently search for "Owner: none()" to find tasks with no owner, but there's no way to search for "Reviewers: none()" in Differential right now.

Add support for this, since it's consistent and reasonable and doesn't seem too weird or niche.

Test Plan: Searched for "Reviewers: none()", found revisions with no reviewers. Searched for "Reviewers: alice, none()", "Reviewers: alice", and "Reviewers: <no constraint>" and got sensible results.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13289

Differential Revision: https://secure.phabricator.com/D20537