Basic framework for fixtures complete

We are obviously missing a bunch of features still, but those can be
added as we move forward. Specifically, we need to address:

  1. adding a setting that automatically runs fixtures every request
  2. implementing field import/export (we don’t even have fields yet)
  3. multilingual slugs (we need to discuss how to store this in JSON)
  4. checking fixture freshness (we have the method, just not
implemented yet)