Added route and checkout state translations

I added the route_translator gem to spree_frontend and so far the test suite is happy.
This is tied to https://github.com/spree/spree_auth_devise/pull/172 and https://github.com/spree/spree/pull/4486

If the feature is legit, I'll need a hint on how and where this should be tested.

This was discussed here https://github.com/jdurand/spree/issues/3