Rename policy capabilities

Summary: Ref T5655. Rename `PhabricatorPolicyCapability` subclasses for consistency.

Test Plan: Browsed a few applications, nothing seemed broken.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin, hach-que

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D10037