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.


+1 -- @dragonfi if you are ok with doing this now, go ahead with @aboudreault 's suggestion, and I will create the repository.

It might however be good to synchronize with @cpennington if we don't want to work toward a more general approach for emitting events from JS for all XBlocks, as this will likely be a common requirement.