Implements stage deployement and cleans up other stage code; adds tests

Has stage deployment create Student objects, select student tracks, and deploy stage urls from the appropriate tracks.

Eliminates unnecessary logic and updates old variable names in ab_testing_tool_app/views/stage_pages.py
Adds test to ab_testing_tool_app/veiws/stage_pages.py
I saw another comment about #20 just as I was leaving today. I didn't realize that the error handling has been moved to the middleware. It looks like that pull request is on hold while you look into Django's built-in error handling. I'm not sure how that should affect this an other pull requests. Let me circle back with folks tomorrow (or later today) and see how we want to proceed.