Email verification and reCAPTCHA signup protection

Added this basic but fundamental protection layer to the website.

The email setup will of course be reused when notifications are eventually implemented.

Currently using SendGrid as the email provider. Very easy to setup, and has a free plan.

Adding reCAPTCHA immediately after email is a must otherwise an attacker could send infinitely many emails to random addresses, which would lead to the domain being marked as spam. I was pleasantly surprised about how easy the integration ended up being.