Insert subresource hints

[jeffkaufman]

We can inform the preload scanner about resources via headers and link tags. We should store what rewritten resources a page uses in the property cache and experiment with hinting them.

See:

• https://www.smashingmagazine.com/2016/02/preload-what-is-it-good-for/
• https://www.w3.org/TR/preload/

[jeffkaufman]

@morlovich has been working on pushing for H2, and in H1 this knowledge about what includes what will probably be surfaced as rel=preload hints.