Fix available class times on course add view

Resolves #151

I've modified the `TimeSelectionForm` to accept a branch and use a default queryset which filters to only active courses in the future. I also added a test for this first (so I could verify that it was failing).

The other changes are largely just to get the code passing in [flake8](https://flake8.readthedocs.org/en/2.0/). flake8 can be great for catching unused variables and helps keep the code consistent with common python code style. Once more files are passing we can add it as part of the travis run.