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
Of course. I guess I should also note that we don't actually control the contents of this field, we just copy whatever segment.io gives us so it may change without notice. The more I discuss this with people, the more I think we should be more explicit in our management of this data, since it is not clear to me that segment.io will protect the continuity of their format. If/when they do change their data format it would be good for us to know sooner than later...