Add a script to mirror a Pombola site's Popolo JSON export to PopIt

You can set up Pombola to regularly output Popolo JSON, and then
afterwards run this script to mirror it to a particular PopIt instance.
It uses the replace-database script so there should be minimal downtime
for the PopIt instance.

This is part of the fix for https://github.com/mysociety/pombola/issues/1429