Allow Herald rules to check for revisions with no reviewers

Summary: Fixes T4225. Adds the NON_EXISTS condition to Herald for "Reviewers", and adds a few more conditions which have reasonable meanings.

Test Plan: Used test console to check a revision with reviewers, and another without reviewers. Both produced the expected results.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4225

Differential Revision: https://secure.phabricator.com/D7757