Hooking the logistration page into the new HTTP endpoint for email opt in

Updating the Login and Registration page to accept the email_opt_in parameter, which will set an organization-wide setting for receiving emails.

Related to the ECOM-525 story, sub task ECOM-680

@rlucioni @wedaly 
I'm testing that from a python point of view, but from the JavaScript, you're suggesting passing in undefined to the EmailOptInInterface function? With the AJAX call mocked out, I'm not sure if that would make a difference?