Make the PhabricatorApplication::getName() method abstract

Summary: In D10027, the getName() method was added to PhabricatorApplication to allow applications to explicitly declare their names rather than relying on the name being inferred from the name of the PhabricatorApplication subclass. I feel that sufficient time has passed such that we can require applications to explicitly declare their names.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11119