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.


This should be `mentoring` not `image-explorer`.

Wondering whether or not it would make more sense as `mentoring.submit` (reflective of the function call) since `attempted` isn't precise (for example, free responses don't really have "attempts").

@zvishal, thoughts?