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