Normalise history cache key URLs, and prevent collisions in boosted anchors that use relative links.

[croxton]

1. Since LocalStorage is bound to the origin (domain/protocol/port triplet), we can normalize the URL format used as a key for a history snapshot to reduce cache misses for the same URL referenced differently. E.g.: https://mysite.com/about, /about and /about/ are normalized to /about.

2. Prevent potential key collisions in boosted relative links, by converting the href to a fully resolved root-relative path. E.g. a boosted anchor shoe.html in the directory products becomes /products/shoe.html.