Simplify the implementation of `PhabricatorPolicyCapability` subclasses

Summary: Instead of implementing the `getCapabilityKey` method in all subclasses of `PhabricatorPolicyCapability`, provide a `final` implementation in the base class which uses reflection. See D9837 and D9985 for similar implementations.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin, hach-que

Differential Revision: https://secure.phabricator.com/D10039