settings: Don't cache stuff in DEBUG mode

In particular, template and view caches can be very annoying when
developing the project.
style: Disable the large screen size

As recommended by @samsmith.
homepage: Make the signup form fill the width better

Addresses some of the issues in #16
style: Change form focus ring colour to green

Fixes #116.
homepage: Add SENDY_URL and SENDY_TOKEN settings