Multi language user interface

This allows people to input data into popit in two languages which must be specified on the about page.

It uses either http-accept language headers or a session based override to set the language the user is viewing and saving in.

Viewing works by applying a decorator to the get function of models that then picks the appropriate language from the returned key and returns that instead of the language object. Saving applies to the body object as it's loaded.
Would be good to redirect the user back to the page they're currently on, rather than always taking them to `/persons`