Add `canonical_path` to `host_paths`

* Calculate `c14n_path_hash` and `canonical_path` at the same time
* `canonical_path` is `NULL`able - we'll make this `NOT NULL` later once populated
Yes; we have to revert the [revert](https://github.com/alphagov/transition/commit/52f0b9fc800a62468ee25d57fe95dec765d1fdc1) to get back the looking up of stuff by `path` (we've backed out of it twice now!)