Mobile events, updates to video events

@stroilova , @mulby , @nasthagiri , @gwprice , @lou-wang , @mhoeber here is a framework that we can work from to get initial documentation out with the release. Notes:
- I am not sure that the same additional context fields that I see for the v.0 play_video (Olga shared with me) event also apply to all of the other video events.
- the event field names for the new edx.app.browser.launched event in Olga's spreadsheet are very rough
Each event emitted in the new format should issue a 'name' field, storing the type (or name) of event. Previously, events had been labeled with an event_type field. The event_type field is being backwards-compatibly deprecated in favor of this new name field in all new implementations of events. We would later go back and clean up the legacy browser events, so that they are also in this updated format. I'd be happy to talk more in person here if that would be better.