Rename PhabricatorPolicyRule subclasses for consistency

Summary: Ref T5655. Fixes T6849. This is another take on D11131, which was missing the DB migration and was reverted in rP7c4de0f6be77ddaea593e1f41ae27211ec179a55.

Test Plan: Ran ./bin/storage upgrade and verified that the classes were renamed in the phabricator_policy.policy table.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6849, T5655

Differential Revision: https://secure.phabricator.com/D11166