Remove path_hash everywhere

* Add a `canonical_path` to `host_paths` to compensate
* Leave `path_hash` fields in DB for now, but make them `NULL`able
We *could* do it in Ruby. Would be on the slow side for 15m rows for a corner case that we'll know about quickly. Effectively, I'm asserting that we're going to know if we want to roll it back just after we've deployed and before the second deploy removes the `path_hash` and `pgcrypto`, and that `down` has a very short shelf life.