Email verification flow

[chouseknecht]

To complete the email notification preferences work, we'll need the backend and UI components to support the email verification flow.

Django Allauth handles sending email verifications. We'll need to understand how it works, and then put the pieces in place on the backend to trigger Django Allauth to send an email verification to the requested email address.

When the user clicks on the verification link from the received email, we'll need to recognized or accept the request, mark the email as verified, and redirect the user to a page indicating the verification succeeded.