Adds a Google reCAPTCHA to the Leaky Paywall registration form. This helps stop spammy sign ups.

It uses version 3 of the Google reCAPTCHA, which detects abusive traffic without any user friction. This means there are no checkboxes or weird characters on the registration form.

The user only sees a small amount of text at the end of the registration form that lets them know the form is protected by reCAPTCHA.