Returns None if course isn't cohorted, even if cohorts exist.  Shouldn't happen on prod, but could if cohorting is turned off mid-course.

caveat: with a bit of hackery.  Need to do better testing once things are live.

- we don't use it, and if we do want to change the arch, we should have a proper central user info service.

racing condition.

- Create new cohorts.py file
- moved code there from models.py
- implemented necessary functions.

Next: testing :)

- still needs style, more features, but basic functionality works

- lets user see and add cohorts
- needs styling
- needs to allow them to actually manage membership!