Change authenticate_user workflow to begin at sign_up

[morganjlopes]

9/10 times, users on early stage apps need to sign_up rather than sign_in. It feels reasonable to prompt them to sign_up first.

Perhaps this could be an admin variable they can change?

https://github.com/plataformatec/devise/wiki/Redirect-to-new-registration-(sign-up)-path-if-unauthenticated