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 are using the same publish_event handler in all XBlocks. Maybe this could be a XBlockMixin in a separated python package (ie. xblock-extensions). We could put more shared stuff in that package through times. This doesn't have to be done now...but to keep in mind. CC @antoviaque