Several fixes/improvements to syncstatic command

This includes:

- Fix for django STATIC_URL formatting (see comments in SHA: 06bceac53 )
- Rewritten syncstatic command that is blazing fast in the optimistic case
- Fix for AUTH_URL setting causing breakage (cherry-picked from rizumu/django-cumulus@a1ab10488)
- Fix for CUMULUS_STATIC_CONTAINER not being used consistently (let me know if this is not the right thing to do and I will back it out)