Rename PhabricatorDashboardPanelType subclasses for consistency

Summary: Ref T5655.

Test Plan: grep

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D11182