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 can add a sentence like "Legacy format, provided for backward compatibility after the addition of the ``current_time`` field." to the description of the currentTime fields. However, if all of the current_time fields are supplied only for event_source = mobile events, it doesn't seem correct to me. @stroilova ?