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.
Since this is a feature flag, shouldn't it be with other feature flags in the `FEATURES` dictionary? It looks like the "MARKETING SITE" section is intended for things like link maps and the cookie name, stuff that the marketing site uses if it's allowed to by the flags in `FEATURES`.