Remove VideoEfficacy experimentation framework

Summary:
As discussed in D9924, this code is no longer used and would be difficult to port to our new A/B testing framework. So, I'm removing it completely. I'm also reworking some of the methods on the `Video` object to make them less hacky and more commented.

This supercedes D10439

Test Plan:
Navigate to http://127.0.0.1:8080/math/arithmetic/addition-subtraction/basic_addition/v/basic-addition

Create a trivial change at http://127.0.0.1:8080/devadmin/content
And then click "publish to website"
Then watch another video to verify that it works

Reviewers: alan

Subscribers: desmond, jace

Differential Revision: http://phabricator.khanacademy.org/D10461