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. 
Does one of these work?

The ``event.event_type`` member field identifies the video events that a browser emits. Video events that the edX mobile app emits include both the ``event.event_type`` member field and a ``name`` field.

When a browser emits a video event, the event contains an identifying ``event.event_type`` member field. When the edX mobile app emits a video event, the event includes both the ``event.event_type`` member field and a ``name`` field.