R1:e397103bf211

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…