Populate `host_paths.canonical_path`

* `HitsMappingsRelations` is already doing it for `host_paths` without a mapping, but not elsewhere
* This covers everything else without having to apply temporary hacks to `HitsMappingsRelations`

This will take as long as it did before, but we can at least deploy it without migrations and run `rake db:migrate` decoupled from the deploy (have improved the progress feedback a little, too). Since it is now not in the same implicit migration transaction as when it was previously lumped in with the [mapping `NULL` change](https://github.com/alphagov/transition/pull/453/files#diff-529c3c39f90bc90d01a4cd42cc8cf3c7R30), it should not cause deadlock.
Is there a particular reason here for using raw SQL and pulling the result out manually, rather than something like `HostPath.where(canonical_path: nil).count`?