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.
I think this is necessary because of the (ugly) way the iframe is currently designed. This uses [`prop`](http://api.jquery.com/prop/), a jQuery attribute added in version 1.6, to get the value of the `checked` property for the first element in the set of elements with id `email-opt-in`. The matching input element has name `opt-in`, and is not contained in a form.

If `checked` is false (i.e., the checkbox isn't checked), then this sets the value of a *second*, hidden input element on the page with name `email_opt_in` to `false`. This input element is contained in the form which is used to POST data from the iframe to the student views.

@stephensanchez, since I'm forced to manage two separate input elements in this way, I don't think I can take advantage of the automatic setting of the input value you're referring to.