Support list-data export and import via web interface

Original LP Author: Sumana Harihareswara , LP Link: https://bugs.launchpad.net/bugs/1414176

Provide a better list-data export and import mechanism via XML. Possibly allow for data export via the web.

This requires discussion and deciding which parts to implement in Postorius and/or HyperKitty.

From http://wiki.mailman.psf.io/DEV/Mailman%203.0?action=diff&rev2=30&rev1=28 -- a TODO for the Mailman 3.x series.