Add configurable ability signup with email only

[imMatt]

As per #3612 the ability to signup with email only, would be beneficial support the addition of logging in using email only and add the ability to use Parse exclusively with emails - hence no usernames ever needed.

[moritzsternemann]

Whats your progress on this?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.