R1:eec2d145df03

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…