Tracking logs

For 'item.picked-up', I did not add a 'location' field, as location is always the list of draggable items.

Instead of 'feedback.closed' I implemented 'feebdack.closed-manually', that only tracks closes via pressing the close button. Feedback is always closed automatically, when the user picks up a new draggable.

Similarly, 'feedback.shown' is not implemented, as 'item.dropped' automatically imply that the feedback is shown, and there is no other way to trigger it.
@antoviaque, yes that's what I was asking about. My feeling is that the xblock is already aware of the `user_id` and includes this as a part of the event context.