Strange behavior:
  - There are 20 cohorts that users should be randomly assigned
    into in HeroesX
  - Almost all, but not all (e.g. ~3300 / 3400) users end up in the
    same group.
  - testing manually locally and on prod in a django shell shows
    nothing wrong

See changes in xml-format.md or wiki (dynamic cohorts v1) for spec.

This reverts commit e4819a1b, reversing
changes made to b6ec41ca.

Now that we're ready to test again, need to revert the revert to put the filtering UI back in.

This reverts commit 66889b87, reversing
changes made to 0dd9c24e.

This was merged too early--it's still missing the filtering and display-your-cohort changes.
Will need to revert this revert later.

update to show cohort name on create and cohort name on view all threads, added cohort dictionary to convert ids to names

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.

- 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!