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
My concern was that the urls are entered by hand leaving plenty of room for a typo. Implementing Django forms should take care of the validation so it sounds like ti will be fixed at a later point. You may have already done so but just in case I would log it so it doesn't fall of the radar.