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.
Nitpick: There's a section in common.py titled 'MARKETING SITE' that this setting should probably be put under.