Deprecate ArcanistBaseWorkflow::getUserGUID in favor of "PHID"

Summary: One day all "GUID" references will be gone, maybe.
Test Plan: grep, ran workflow before changing and got a warning about
deprecation
Reviewed By: aran
Reviewers: mgummelt, tuomaspelkonen, aran, jungejason
CC: aran
Differential Revision: 671