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. 
Not sure what emits the events - the browser, the app? Should we specify the actor here? Suggestions: "These events originate from the browser or the edX mobile app when a user interacts with a video" or "The browser or edX mobile app emits these events when a user interacts with a video." 

Extra space after "or,"