Tracking logs

`feedback.shown` is not implemented, since it is already implied by a `problem.attempted` event. There is no other way to trigger a feedback.

It would be nice to add an `item_id` to the `feedback.closed` event. It might be a good idea, and relatively easy, but that would include giving IDs for each feedback, meaning some extra work and code.


We haven't really thought too hard about client side emission yet. I agree that a change like this would make sense in xblock core.

The only concern I have is that we currently differentiate between "browser" sourced events and "server" sourced events. There are some small details like that we should think through...