Fix all spectacular warnings and add OpenAPI spec to release draft

[jennydaman]

• Most get_queryset overrides need to have an extra two lines of code if getattr(self, "swagger_fake_view", False): return TheModel.objects.none()
• Added a class decorator @core.serializers.file_serializer which unifies the duplicated code between file serializers which provide the fields fname, fsize, file_resource, owner_username, parent_folder, and owner