Move admin promotions to modular transactions

Summary: Continue clean up of super-old code. I am pretty proud of "defrocked", but would also consider "dethroned", "ousted", "unseated", "unmade", or "disenfranchised". I feel like there's a word for being kicked out of Hogwarts and having your wizarding powers revoked, but it is not leaping to mind.

Test Plan: Promoted/demoted users to/from admin, attempted to demote myself and observed preserved witty text, checked user timelines, checked feed, checked DB for sanity, including user_logs. I didn't test exposing this via Conduit to attempt promoting a user without having admin access.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19891