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.
Style nitpick: could you please move the check for the feature flag to the caller of this function?  I think it's clearer to say "don't call this function" than "call this function which does nothing".