Redis Cache

Following the changes in #365 (closed) and #362 (closed) , we want to consider implementing a cache which optionally uses Redis. Optionally to avoid the dependency

Needs investigation

Probably implemented using a new DjangoLDP package, overriding the LDPSerializer. A priori it would need a server setting and DjangoLDP changes to how the cache is reset in Model, which will be tightly coupled to LDPSerializer when the issue is taken up