Rewrite Fetch integration, especially around redirects

This cleans up the Fetch integration in the following ways:

- Integrate the handling of Referrer-Policy headers on redirects with
  the HTTP-redirect fetch algorithm. This (I think) removes the need for
  responses to have associated referrer policies, and is also more
  precise/clean.

- Insert a TODO to define an HTML integration for the referrerpolicy
  content attributes. This removes the need for handwavy handling of the
  attribute when determining the Referer for a request.

- Use the request's referrer policy when determining the Referer for a
  request (no need to consult the environment's policy because that is
  now accounted for in Fetch as of
  whatwg/fetch#280). Issue 26