[bug 979266] Update celery to 3.0.24

A bunch of commits here, mostly moving libs from `vendor/packages` to `vendor/src` (is that OK?).

After all was said and done, to verify:

* Run tests
* Switch `CELERY_ALWAYS_EAGER = False`
* Fire up celery `./manage.py celerdy`
* Verify that three tasks are registered (you'll need `CELERYD_LOG_LEVEL = logging.INFO` for that to show up)
* Fire up the devserver and submit feedback. Verify that the indexing task works in celery.
* Go to the admin and trigger a reindex. Verify that works in celery too.

I think that's it. r?