Don't explicitly name abstract base classes

Summary:
Ref T5655. It is superfluous to include "base" in the name of an abstract base class. Furthermore, it is not done consistently within the code base.

In order to retain compatibility with external code, I have kept the ArcanistBaseWorkflow class (which trivially extends from ArcanistWorkflow), but it is now deprecated and should output a warning message. Similarly for ArcanistBaseUnitTestEngine.

Test Plan: Created a workflow which extends from ArcanistBaseWorkflow. Executed the workflow and saw a deprecation warning.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin, aurelijus

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D9983