Implement shim for mobile video events.

This PR addresses the following issues:

1) All requests return a 200 OK unless there is an authorization failure. This is deliberate in case the secret key is compromised.
2) Push all of the nasty logic necessary to generate compatible video events into the LMS instead of trying to do that mapping on the mobile device
3) Stop using the deprecated "action" field in the segment.io event. According to their support team this field should not be used anymore.

Fixes: AN-3818
2 questions:
1. Why is this specific to Mobile?  This translation is applicable to any app that generates analytics events that need to support legacy formats.
2. Why is this specific to only video events?  Are there not other legacy events that we need to continue to support that have the same legacy format requirements?