title: Accept username OR email for logging in with django-allauth
body: <p>I see django-allauth supports forcing users to login using their email address, and doesn't ask them for a username when signing up (instead generating one automatically from the email address) - <a href="https://stackoverflow.com/a/19683532/221001">https://stackoverflow.com/a/19683532/221001</a></p>
  
  <p>Is it possible to have a user sign up, entering an email address and username manually, and then allow them to sign in using either? (e.g. there are two fields on the Login page: "username or email" and "password")</p>