R1:3ef0721adacc

Reduce PhabricatorUser::getOmnipotentUser calls by adding a getViewer method to PhbaricatorDestructionEngine

Summary:
Fixes T6956. Before this change, we called PhabricatorUser::getOmnipotentUser in the various delete methods to query the data. Now, we use $engine->getViewer(), since its always a good thing to have less calls to PhabricatorUser::getOmnipotentUser thrown around the codebase.

I used the "codemod" tool to audit the existing calls to PhabricatorDestructorEngine (all of them) so ostensibly…