R1:94ac3c084b27

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