R1:39d415e90eb3

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…