Heroku fixes.

This gets Perma working on Heroku, with mirroring. In the process, that involved updating how we launch celery; switching mirroring code to use default_storage; and refactoring settings.

When deploying:

- pip install -r requirements.txt
- make sure that your settings.py file refers to .deployments.settings_foo instead of just .settings_foo
- If you have a custom setting for MEDIA_ARCHIVES_ROOT, change it to be relative to MEDIA_ROOT
- if you're running celery as a daemon, update it to use "celery -A perma" instead of "python manage.py celery". An example is in services/celery/celery.conf.