Remove `path_hash` everywhere but the DB

This is a re-opened #452 (Github won't allow a reopening where the branch has seen a force-push).

`Transition::Import::HitsMappingsRelations#connect_mappings_to_host_paths!` only [writes to mappings where `mapping_id IS NULL`](https://github.com/alphagov/transition/blob/4e55cf3bf5a6019fcc620b072e69250aba6892a5/lib/transition/import/hits_mappings_relations.rb#L33). The migration now uses its own [`populate_canonical_paths!`](https://github.com/alphagov/transition/pull/453/files#diff-529c3c39f90bc90d01a4cd42cc8cf3c7R5) with batch sizes of 10K / ~650K to fill paths as fast as a Ruby solution is able.

### DB

* Leave `hits` and `mappings` `path_hash` `NULL`able so that existing
  processes are not disrupted
* Add `canonical_path` to `host_paths` to replace `c14n_path_hash`
* Populate it using the private
  `HitsMappingsRelations#connect_mappings_to_host_paths`
* Move the UNIQUE `host_paths` index from `(host_id, path_hash)` to
  `(host_id, path)`
* Cope with any possible rollback (including backfilling `path_hash` for `hits`
  and `mappings` in the migration's `down`)

### Code

* Remap usages of `host_paths.c14n_path_hash` to
  `host_paths.canonical_path`
* Replace usages of `path_hash` with appropriate plain
  `*path` alternatives
One question (and I’m not entirely sure how much this matters): why is this validating uniqueness of `site_id` in the scope of a `path`, rather than validation uniqueness of `path` in the scope of a `site_id`? The latter seems to reflect the intention more accurately, but I’m well aware that I don’t know the intimate details of Rails’ uniqueness validation.