Move users to modular transactions

Summary:
Ref T13164. See PHI642. I'd like to provide a third-generation user.edit API endpoint and make user.enable and user.disable obsolete before meddling with policy details, even if it isn't full-fledged yet.

Users do already have a transactions table and a Transaction-based editor, but it's only used for editing title, real name, etc. All of these are custom fields, so their support comes in automatically through CustomField extension code.

Realign it for modular transactions so new code will be fully modern. There are no actual standalone transaction types yet so this diff is pretty thin.

Test Plan:

• Grepped for UserProfileEditor.
• Edited a user's title/real name/icon.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19576