Beginnings of creating mappings from HostPaths
More tests for creating mappings from HostPaths
CreateMappingsFromHostPaths is limited to one site
HostPath can now set path_hash and c14n_path_hash

Mostly copied from Mapping.

This simplifies creating HostPaths in tests. Unfortunately it duplicates code
in Transition::Import::HitsMappingsRelations as there isn't an obvious way to
refactor that class to share this code.
Refactor to reduce unnecessary queries
Add rake task to create mappings from HostPaths