Facebook auth without an email should allow user to enter email

In some cases Facebook doesn't send back a user's email. In this
case, allow the user to enter their email address.

See
https://meta.discourse.org/t/facebook-initial-login-create-account-dialog-leaves-email-field-blank/13815/15
Merge pull request #3030 from longhotsummer/fix-facebook-no-email

Facebook auth without an email should allow user to enter email