Transitive third party exceptions

This is meant to satisfy ACTION-194 and is a proposal for transitive third
party exceptions. I'm not sure if it's necessary if we restrict things to
"first-party/*" but if you want to list out "first-party/third-party"
explicit/explicit exceptions, I believe it would be necessary for things
like advertising networks to function.

"If a third party has been granted an exception on a page, then any
resources fetched by that third party, including items such as images
included by that third party, content dynamically fetched by that third
party, or another third party that is redirected to (such as via an HTTP
302 status code) are considered to be covered by that exception. This
applies transitively, meaning that if in a given context "Site A" is a
third party and has an exception, if it redirects to "Site B" then "Site B"
is covered by that exception, as would "Site C" if "Site B" either included
content from or redirected to "Site C".

-Ian