Split Testing

This PR gives us the ability to perform split testing on the LMS. For details, see the [documentation](https://github.com/edx/edx-platform/wiki/Split-Testing). The changes in this PR will require DevOps to perform a migration to create the necessary tables, as well as clearance from them to activate the Django admin site on stage and prod. I'm currently speaking with them about the best way to do this.

The experiments to be performed on the LMS high-level tabs are verified, but have not been formally tested. This is because the experiments can be turned off instantly (without pushing new code), and the best-performing variation will be thoroughly tested before being added permanently to the codebase.

@cpennington and @singingwolfboy, could you please help me review this? (N.B. Although my internship ends on August 30, I'm continuing with edX on a limited hours basis during the school year, so this doesn't need to be reviewed and merged right this minute.)