Return 404 for course about pages if the marketing site is enabled

It would be better to redirect to the marketing site course about pages, but
the URLs for those are not easily computed, and we do not have time to set up
a full mapping in the LMS config.

lms/djangoapps/courseware/views.py

@@ -515,6 +515,9 @@ def registered_for_course(course, user):
 @ensure_csrf_cookie
 @cache_if_anonymous
 def course_about(request, course_id):
+    if settings.MITX_FEATURES.get('ENABLE_MKTG_SITE', False):
+        raise Http404
+
     course = get_course_with_access(request.user, course_id, 'see_exists')
     registered = registered_for_course(course, request.user)