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
Hi @nasthagiri  , neither of these fields are the edX video ID, unfortunately. It would be good to capture the edX video ID as well. The id field is a legacy field that html-escapes the module id, emitted in browser events only. I am hoping the browser events will be updated to also emit the module_id. You are right, it would be good if both browser and mobile events emitted the edX video identifier as well, exactly for the reasons you provide here. We can talk in person more if you have questions about the legacy event.id field.