Standardize referrer policy

Hey,

I'd like to propose to create a more formal standard for referrer policies.
Until now, there is just a wiki entry at whatwg (
http://wiki.whatwg.org/wiki/Meta_referrer) and various other specs (CSP,
fetch) refer to referrer policies, however, there is no formal standard.

With a lot of help from Mike, we've put together a first draft here:
https://w3c.github.io/webappsec/specs/referrer-policy/

Any comments are more than welcome!

best
-jochen