Standardize types used for logging events from the instructor dashboard

Information previously sent through the `event_type` string is now sent in appropriately named fields within the `event` dictionary, which is what it was designed to be used for. These changes will be cataloged/documented in my LMS Event Inventory wiki page, and will merge with Miles' refactor. @dianakhuang and @shnayder, we spoke about this earlier - could you please review?