Adds video events emitted from the mobile app

@stroilova , @nasthagiri , @mulby please review ASAP. This is all new. I have integrated descriptions of the mobile-sourced events in with the browser-sourced events, as @stroilova recommended, rather than adding them in a separate section (which you reviewed last week). @stroilova , in the interests of time, I was not able to redesign this section to reduce repetition. On the other hand, with @mulby's help I confirmed that the event.id field is present for both mobile-sourced and browser-sourced video events.
@mhoeber , @srpearce , FYI this has to be published tomorrow. 
Quite a bit of passive voice in the following - I'm going to stop flagging it because you might have a good reason for using it, though. 

If there isn't a specific reason to use it, I'd change it to help with GE.