major: test suite, fixed bug with InvoicePermissions

I haven't tested whether this fix totally solves components/sib-invoicing#24 (closed) on the demonstration, but it certainly solves the permissions side of it

I've also added some unit tests for the package and added these to the pipeline... @balessan I think there's another step you need to do in the pipeline?

This update is major because I update the dependencies to djangoldp>=2 and djangoldp_project>=2 which it references