Implement a Referrer view

[jverkoey]

This view type would be a simplification of the meta referrer tag, defined in https://html.spec.whatwg.org/multipage/semantics.html#standard-metadata-names.

The referrer policy meta attribute must be one of the values defined by the W3C's standard, making this a good candidate for packaging up in a view that accepts an enum of one of the accepted values.