Remove path_hash everywhere

* Add a c14n_path to host_paths to compensate
* Use canonicalised path instead
I think I did consider `canonical_path`, but it wasn't clear *whose* canonical. Canonical URLs are a site-specific thing, and I didn't want to give the impression that it was (even though a path, not a URL), for example, decided by DFE's legacy CMS. Equally, I didn't want to give the impression that it was the local filesystem kind of [canonical](http://stackoverflow.com/questions/12100299/whats-a-canonical-path). Could've said `site_canonical_path` and that might have been more self-evident. Now, though, people are used to seeing `c14n_something` on `HostPath`, so it just has muscle memory (and still brevity) to recommend it.