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 
Heya, thanks for the input Renzo! 

* I'm fine with the names course_id and opt_in. Sounds good to me.
* By the UI design, "True" is the default setting for opting in, so I made that the default value when False is not explicit. For the sake of having uniform contracts, I'll go with your default = false to avoid confusion.
* My understanding of DATA vs. POST is that DATA is offered as a useful utility by the Django REST framework to parse more parameters than just the POST form data. http://www.django-rest-framework.org/api-guide/requests/  By design I always use this over POST.
* On line 874 I check that the opt_in and course_id parameters are set via a decorator, or the function throws an exception before it is even executed.  At this point I would prefer the code to fail loudly if it somehow went beyond this call. To that note, I can add a unit test that will fail if anyone tries to remove the decorator.