Extend all "ManagementWorkflow" classes from a base class

Summary:
Ref T2015. Not directly related to Drydock, but I've wanted to do this for a bit.

Introduce a common base class for all the workflows in the scripts in `bin/*`. This slightly reduces code duplication by moving `isExecutable()` to the base, but also provides `getViewer()`. This is a little nicer than `PhabricatorUser::getOmnipotentUser()` and gives us a layer of indirection if we ever want to introduce more general viewer mechanisms in scripts.

Test Plan: Lint; ran some of the scripts.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D7838

src/applications/cache/management/PhabricatorCacheManagementWorkflow.php

@@ -1,10 +1,6 @@
 <?php
 
 abstract class PhabricatorCacheManagementWorkflow
-  extends PhutilArgumentWorkflow {
-
-  final public function isExecutable() {
-    return true;
-  }
+  extends PhabricatorManagementWorkflow {
 
 }