R1:fff08a9894bd

Allow emails to be used for login

Summary:
Despite the form's claims that you can login with username or email, it actually
accepted only username.

Test Plan:
Logged in using my email.

Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran
CC: jr, anjali, aran, jungejason
Differential Revision: 354