Replace Serializer cache with Django caches

Django's caching system is much more configurable than ours

We could reuse their code to provide SERIALIZER_CACHES (and ACTIVITY_CACHES), allowing our users the same degree of control over the caching system as they receive in Django

The places to begin the investigation of that:

• https://github.com/django/django/blob/stable/2.2.x/django/middleware/cache.py#L156
• https://github.com/django/django/blob/stable/2.2.x/django/core/cache/__init__.py#L57