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
Display an estimate of the time remaining.

This will be a fairly crude estimate, and won't be particularly accurate
if the migration speeds up or slows down as populates more rows, but
it'll at least give us an indication of how long we've got to wait.
Merge pull request #461 from alphagov/populate_canonical_path

Populate `host_paths.canonical_path`