Allow site import to move hosts between sites

This is useful when we want to split some hosts to a new site.

It should make deploying this: https://github.com/alphagov/redirector/pull/705/files easier too.
It's probably a good idea to also test here that `other_mapping` now has a `hit_count` equal to `hit.count`, which should have been set by `HitsMappingsRelations.refresh!`.