Feature/track weights

This feature allows admin users assign weights to tracks and have students randomly assigned by weighted probability. 
Note: 105 lines out of 235 are HTML template changes. The rest are backend changes.
Dependencies: feature/assignment_method
- Assigning tracks portion of deploy_intervention_point has been updated and moved to 
controller.assign_track_and_create_student
- New model to store weight 
- New URLs and appropriate backend pages to render new HTML
- New exceptions
Yes. http://legacy.python.org/dev/peps/pep-0328/#rationale-for-parentheses
It's mostly to avoid the backslash continuations.