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
Only create mappings for sites Transition manages

The canonical source for mappings for these sites is still redirector.
Sites with global statuses generate a warning

At the time of writing, mappings are never used for sites with a global HTTP
status.
Provide feedback when creating mappings
Ask for confirmation if site has a global status

Only showing a warning doesn't give the user time to react, so also
ask for explicit confirmation before continuing.
Rename task and class without 'create'

'import' is the verb, so 'mappings_from_host_paths' is sufficient.
Rename method to 'refresh!' instead of 'call'

This indicates that the operation is idempotent.
Use 'z' everywhere in 'canonicalize'
Point out that new mappings won't be connected
Merge pull request #220 from alphagov/create_mappings_from_host_paths

Create mappings from HostPaths