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
@stroilova should confirm this for me. I think the state is that on mobile devices we emit a "module_id" field and the browser emits an "id" field in a different format. We would like to add the module_id field to the browser events at some point.

The "id" field emitted from the browser contains an identifier that is not very helpful, while the "module_id" field contains a much more useful identifier for the video.