Serializer: handle recursion while suppressing cyclic references.
Hard-code serialization of related models in model-specific serializers.
Handle None-valued average price fields.
Add URIs to API response. Hide fewer fields behing 'include_related' flag.
Add API endpoints for Product, Procurement, Manufacturer and Supplier models.
Merge branch 'serializer' into refactor
Add product catalogue to Supplier detail endpoint.
Add 'alternative_products' list to Product detail endpoint.
Add 'procurements' list to Medicine detail endpoint.