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
No strong insight. My preference is floats. 100ms resolution is adequate, but there's no reason to not have maximum precision. UTC is good. 

What's even better is standardization. In other words, sending out exactly what we were doing before. Breaks in format are painful. They lead to broken code, incorrect results, etc. depending on pipeline, and you often don't find out until many hours in. Legacy or new events in an inconsistent format are not good.