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. 
We are actually using their event collection system on the mobile device. It provides a lot of features related to batching up events when the device is offline and transmitting them all at once when connectivity is restored. It also dives into the phone and gathers a bunch of this information for us.

Perhaps something like: "Includes member dictionaries and fields for device specific data gathered by the event collection library. Note that since this library is provided by a third-party, edX does not have direct control over the contents of this field and the contents may change without notice."

Or something along those lines.