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

src/applications/oauthserver/capability/PhabricatorOAuthServerCreateClientsCapability.php

@@ -5,10 +5,6 @@ final class PhabricatorOAuthServerCreateClientsCapability
 
   const CAPABILITY = 'oauthserver.create';
 
-  public function getCapabilityKey() {
-    return self::CAPABILITY;
-  }
-
   public function getCapabilityName() {
     return pht('Can Create OAuth Applications');
   }