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