Prep marketing iframe and relevant courseware view 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.

@stephensanchez and @wedaly, could you please review this when you're able? @AlasdairSwan, FYI.
Totally valid concern; I've done this. However, since the only arguments currently passed to this decorator are GET parameters, I've modified it to accept an arbitrary number of arguments which it considers to be GET parameters, instead of a list.