Remove path_hash everywhere

* Add a c14n_path to host_paths to compensate
* Use canonicalised path instead
There wasn't - when it was `c14n_path_hash` it was largely for brevity (and, I suppose, to try and associate it with the c14n in [optic14n](https://github.com/alphagov/optic14n)). Yes, precalculated using `Site#canonical_path`.