bugfix: fixed permissions issues for sib-invoicing

Closes components/sib-invoicing#24 (closed) Closes components/sib-invoicing#20 (closed)

invoicing module is not suitable for a production environment. This MR fixes the issues described and makes it work for the demo

There are some workarounds to be aware of:

• SERIALIZER_CACHE must be set to False (djangoldp#341 (closed))
• Batch/Task permissions have been set to "is authenticated" (#11). This effort should wait for DjangoLDPv2.1 (djangoldp!193 (merged)), there is a known issue in the old permissions system where the child container is given the parent's id (this was one of the bugs causing the issues referenced)
• changes need to be made for federation (#10 (closed))