Updated context processor for CDN and staticmedia

Adds an additional context processor that conditionally updates the STATIC_URL context variable. It's toggled by DEBUG or by an explicit setting in the CUMULUS dictionary. Includes documentation update.

In concert with this:

* Updates the containerinfo management command to use the CUMULUS settings, vs. the 'old' CUMULUS_ settings.
* Updates the syncstatic management command to use the AUTH_URL from CUMULUS, which the settings file already identifies.