Auto GUID enhancements

[cstirdivant]

          > You must enable auto GUID generation for your PRs by following the instructions here https://github.com/google/digitalbuildings/blob/master/CONTRIBUTING.md#enabling-auto-guid-generation-for-prs

Auto-generation does not work for your first PR (you submitted your first PR last week) -- if the GUIDs aren't automatically added, please add manually by generating an UUID4 (https://www.uuidgenerator.net/) and inserting into your branch.

Looks like the GUIDs are only generated on pushes to a branch called master. Is there a reason that the logic doesn't allow GUID generation on pull-requests to your master branch? I don't mind producing another PR to expand this to improve outside engagement, but I'm weary of stepping on toes (or there could be a darn good reason not to that I'm unfamiliar with).

Originally posted by @ohshazbot in #1254 (comment)

Good questions -- we will need to dig into the original technical design documents before proceeding. Opening as issue to track.