Updating the marketing iframe, courseware views, and student views for email opt-in

Feature flagged where necessary. This puts a checkbox in the marketing iframe. The iframe uses an `organization_full_name` parameter forwarded from Drupal by the courseware views and POSTs an `email_opt_in` parameter to the student views, preserving it on 403.

Related to the ECOM-525 story, subtasks ECOM-681 and ECOM-686.

@stephensanchez and @wedaly, could you please review this when you're able? @AlasdairSwan, FYI.
If this code was going to be here in the future, I'd agree that moving the code to its own file would be good. However, since we intend to remove these iframes in the near future, and this part of the email opt-in work is only meant to introduce minimal changes, now may not be the best time to make the change. Do you agree, or would you prefer to see the inline JS pulled out regardless?