Read only fields

Currently DjangoLDP model fields can't be made read-only by the serializer or the admin panel

The easiest way to make a field read-only in the serializer is to extend it and set DRF's read_only_fields

In the past (with auto_author) this was achieved by overriding get_default_field_names: https://git.startinblox.com/djangoldp-packages/djangoldp/blob/5f6b29fd323bc835d77bc8ceadee09a0646f69a8/djangoldp/serializers.py#L274

For the admin case we could create a DjangoLDPAdmin and override get_readonly_fields (overriding this instead of using the built-in readonly_fields to allow users to add their own fields without breaking anything)