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'm not really familiar with the best practices regarding integration tests. I was thinking it would be more efficient and cleaner to have the database calls in a setup method. But maybe that doesn't work in these cases?