Add non-JS Child Benefit Calculator

This calculator _is not finished_, but this pull request contains a lot of work and I'm doing it now to avoid the future pull request being silly big and so it can go up on preview for others to see.

This PR adds the child benefit calculator logic & views and a non-JS working version of the form.

Still work to be done, but this is a good base and it would be good to get it on preview so that others can see it.