Remove path_hash everywhere but the DB

* Add `canonical_path` (with index) to host_paths to compensate
* Change NOT NULL path_hash fields to NULLable (or we'd still have to calculate them)
* Cope with any possible rollback in the migration's `down`
Merge pull request #449 from alphagov/remove-path-hash-from-transition-entirely

Remove path_hash everywhere but the DB