Add the POST endpoint and the infrastructure to allow the calculator form to be developed

Merged Philip Elson requested to merge feature/calculator-infra into master

This puts in-place the form.html static file and the server side processing of the form from this page. The results from the server are then put into the page.

This is the basic framework by which we will deliver the COVID calculator app, with @gaazzopa working on the front-end (and report presentation) and @mrognlie working on the backend (and report presentation).

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