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. 
Take or leave (trying to avoid passive voice):

When a user interacts with a video file offline by using the edX mobile app, the app can only forward the corresponding events during the next connection opportunity. As a result, the value for ``currentTime`` can be different from the date for ``context.received_at``. Data packages can include events from past dates.