Add tracking on expand of upsell message

This adds tracking on click/expansion of the upsell message appearing on the student dashboard, as per [ECOM-292](https://openedx.atlassian.net/browse/ECOM-292). I've also ripped out some relevant Django Waffle code; I'll get the rest in a future PR.

@flowerhack and @wedaly, could you give this a once-over? I've manually tested the emission of the event.