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.
I'm concerned about coupling this utility decorator with the marketing iframe view.  I think you should generalize this so that the decorator accepts a list of GET parameters to include in the querystring.  The marketing view can then pass `[organization_full_name]` to the decorator.