Disable browser autocomplete from 2FA textfield.

The ability to do this as per [this article](http://blackbe.lt/adding-custom-attributes-emberjs-form-input-elements/) has already been added to the dashboard:
https://github.com/balanced/balanced-dashboard/blob/9cbf8cb96006ec63094baffd51c5ab62a7816356/app/lib/forms.js#L3

:+1: