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

Puts a checkbox in the iframe. The iframe uses an organization_full_name parameter sent by the courseware views and POSTs an email_opt_in parameter to the student views, preserving it on 403.

Add tests, modify view to receive organization full name from GET parameters
Merge pull request #6043 from edx/renzo/iframe-prep

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