Add event tracking endpoint to be called by segment.io

As discussed, we are planning on turning on the "webhook" integration in segment.io and pointing it at this endpoint in order to gather      events from the mobile devices that will *only* be publishing events to segment.io. Since we will be "drinking from the firehose" of events  produced by the LMS, we drop on the floor events we can't handle (identify, alias etc) as well as events that did not originate from mobile  devices.

Outstanding concerns:

* This will significantly increase our LMS request volume since we emit segment.io events on most page views. Most of these requests should be handled very quickly (they short circuit out at the top of the method). However, they will incur the full overhead of a request.
* Not sure the mobile apps will have access to the user_id of the logged in user.
* Mobile apps will need to annotate the events at emission time with the course_id they are associated with.

@ormsbee, @rocha, @brianhw
One day we can add something like a `is_valid` and a `course_key` field, to help us figure out things that could go wrong.