Provide an "Add blocking reviewer..." Herald action

Summary: Ref T1279. These reviewers don't actually create a logical block yet (that is, revisions still transition to "accepted" even in their presence), but this handles everything except that.

Test Plan: Added Herald rules and updated revisions; see screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7244