Allow specifying response as a reference

[vmax]

We're trying to support a usecase of defining responses in one place and then referencing them in the schema. This is useful for specifying generic errors (e.g. 500 is always "Server Error"). Examples from resulting OpenAPI YAML fragments:

responses:
  NotFound:
    description: "Requested object not found."
  ServerError:
    description: "Internal server error."

... 
paths:
  /example_resource/:
    get:
      # ... omitted for brevity
      responses:
        404:
          $ref: "#/responses/NotFound"
        500:
          $ref: "#/responses/ServerError"

drf_yasg custom code would look like this:

class ResponseRef(_Ref):
    def __init__(self, schema_name):
        super().__init__(None, schema_name, 'responses', None, ignore_unresolved=True)

@method_decorator(name='list', decorator=swagger_auto_schema(
    responses={
        404: ResponseRef('NotFound'),
        500: ResponseRef('ServerError'),
    },
))
class SomeViewSet(ListModelMixin, BaseApiViewSet):
    # ...