Export data for a Collective

[cb109]

There should be a way to have a simple human- and machine-readable backup of all data around a Collective, preferrably as JSON. Django has a generic serializer that may help with that and there's also ways to aggregate related data as a whole https://cbuelter.wordpress.com/2016/10/26/collecting-related-objects-in-django/

This is a prerequisite before even thinking about opening this up to the public on a shared server.