Prep marketing iframe and relevant courseware view for email opt-in

Feature flagged. Puts a checkbox in the 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.
Update the student views to accept the email_opt_in parameter

Includes tests checking that the student views integrate with the profile API as expected.
Merge pull request #6062 from edx/renzo/email-opt-in

Updating the marketing iframe, courseware views, and student views for email opt-in