Move main form from root to /register

This moves the main form off of the root url and serves it on `/register`.

The GOV.UK service manual [mandates](https://www.gov.uk/service-manual/operations/operating-servicegovuk-subdomains.html#robotstxt-and-root-level-redirections) a root-level redirect to the entry point on www.gov.uk

A convenience redirect from root to `/register` exists to make dev less irritating; the GOV.UK requirement should happen in production outside of this app.