Remove items from the shopping cart if they are for a course whose course enrollment window has closed

changed the url name to verify_cart and change the color of the message in the shopping cart
Studio: adding UI and styling for component-level access modal
fixup! addressing UI review/hand off feedback:

* removed summary header UI element
* change "access" to "visibility"
* change second radio element to "Specific Content Groups"
* replaced key icon with eye icon
* synced up specific visibility-related styling (sidebar + active icon state)
* synced up all developer (NOTE: @andyarmstrong...) note mentions