add audit trail for user management changes
Original LP Author: Sumana Harihareswara , LP Link: https://bugs.launchpad.net/bugs/1414174
Add audit trail for user subscriptions. Who added them? What checks were made?
From http://wiki.mailman.psf.io/DEV/Mailman%203.0?action=diff&rev2=30&rev1=28 -- a TODO for the Mailman 3.x series.