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 
Okay, using DATA over POST seems reasonable; thanks for the link. I see the decorators you mentioned, too. Adding an extra test which fails if someone removes them sounds like a good idea.