Enable drag & drop from course index to course content and viceversa.

Manual testing:

1. Login as admin and create a topic format course with some activities and sections
2. Access the course and enable edition.
3. Display the course index and uncollapse some of the topics with activities (you probably need to close the main drawer to display the course index)
4. Start dragging an activity from the course content:
   1. Check the element is dimmed in the course content while you drag it 
   2. Check when you move over other course content activities a black line appears indicating the drop position.
   3. Check when you move over a section title it highlights the section title as a dropzone
   4. Check when you move over a course index activity it shows a black line representing the drop position
   5. Check when you move over a course index section title it highlights as a dropzone.
5. Drop the element in the page-top bar (where the instance title is)
   1. Check that the element is not dimmed anymore.
6. Drag&drop the activity from the content to somewhere in the course index
   1. Check that a few seconds later the activity has changed its position in both course content and course index.
7. Repeat the same test but with an activity from the course index (and drop it on the course content)