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
We've still got the index on [site_id and path_hash](https://github.com/alphagov/transition/blob/master/db/schema.rb#L103) so I think this should be fine.