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
I would like to actually move to alpha order for member fields, as I do not have access to events and cannot reliably get info on the actual sequence of member fields. 
Alphabetizing all member fields in this file is outside of the scope of this story, however.