Require one-time email verification when signing in

[allanlasser]

To enhance our sign in flow and improve security for user accounts, we should prompt users to confirm their email if it's unverified.

1. When signing in, check if the user's email is verified.
2. If it is unverified, send them an email with a copyable verification code.
3. Redirect the user to a page with a Verification Form, styled to appear as a sign-in step, and making sure to pass along the next query. This page should have an action for resending the code. It should not reveal the user's account email.
4. After the user provides the correct code, redirect them to the URL in the next query param.