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 don't know whether JSON has guaranteed fields. Some video events in the browser used current_time, while others used currentTime. The goal here is to standardize, move to underscore_delimited identifiers (current_time), and emit the legacy currentTime field whenever it was emitted before.