Summary: Stripe Checkout adds 'Sign up' option

Why It Matters: This change makes it easier for new users to start using Stripe Checkout.

What Changed:

• The "Sign in" link was changed to "Log in".
• A new "Sign up" link was added next to the "Log in" link.
• The "Start now" button now links to a registration page for Checkout.
• A "Try the demo" link was added next to the "Start now" button.