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 
Continuing in the same vein as above, I'd change `optIn` to `opt_in`, to be consistent with our other parameter names. Even better, why not use `email_opt_in`? This makes it clear what's being opted-in to, and is also consistent with the GET parameter's name.