Allow Herald rules to add reviewers

Summary:
Ref T1279. Although I think this is a bad idea in general (we once supported it, removed it, and seemed better off for it) users expect it to exist and want it to be available. Give them enough rope to shoot themselves in the foot.

I will probably write some lengthy treatise on how you shouldn't use this rule later.

Implementation is straightforward because Differential previously supported this rule.

This rule can also be used to add project reviewers.

Test Plan: Made some "add reviewers" rules, created revisions, saw reviewers trigger.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7235