minor: optimize permissions queries

Closes #298 (closed)

This MR potentially introduces breaking changes for custom LDPPermissions subclasses

Made this a minor update to reflect that

Documentation:

• release notes have overview of changes & what it means for package developers
• LDPPermissions guide now has a new section for writing FilterBackends to be used in the LDPViewSet nested serializer

DjangoLDP-Circle tests at least are failing, and other packages which use custom permissions classes will need some changes too