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.