Small changes to the segment.io event handler

Fix a couple of outstanding issues discovered during end-to-end testing:

1) The segment.io client libraries don't always set the channel field properly, allow it to be overridden.
2) Parse out the userAgent from the segment.io event and put in the normal location it is found in tracking log events.
3) The userId provided in the segment.io event is not necessarily an integer, so ensure we cast it to one before saving it out to the tracking logs.

Reviewers: @rocha, @nasthagiri
FYI: @brianhw, @stroilova, @gwprice
Does `User.object.get(pk=user_id)` works correctly if `user_id` is a string? I presume it does looking at the code here, but it may be worth to double check. Django magic.