[POC] Implementing alternate url / translation link for engines

Just a proof of concept, a suggested approach to implementing lookup of alternate urls for engines.

Quite happy with the approach to overriding the locale when generating routes, not so happy with the way of encapsulating the ids, etc.

I have a feeling though that we're going to need to build out the api to pull back tool information from public_website and so if that's the case I imagine we'll end up getting a lot of this from there.
This shouldn't refer to the pensions calculator if its in a "general" controller like this. Maybe a subclass is needed? Or something route aware?