Add abstract getGroup() method

Summary: In D11722, a `getGroup()` method was added to all subclasses of `PhabricatorApplicationConfigOptions`, but no abstract method was added to the base class. This will fail if a custom `*ConfigOptions` class does not provide a `getGroup()` method, in which case `$group->getGroup()` (in `PhabricatorConfigListController`) will fatal.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11756

src/applications/config/option/PhabricatorApplicationConfigOptions.php

@@ -4,6 +4,7 @@ abstract class PhabricatorApplicationConfigOptions extends Phobject {
 
   abstract public function getName();
   abstract public function getDescription();
+  abstract public function getGroup();
   abstract public function getOptions();
 
   public function getFontIcon() {