Stop settings.CACHES warnings from bring printed at startup

Anyone on serverlog@learningu.org has probably noticed that we get an e-mail every time the dbmail cron job is run, rather than every time it sends e-mail.  This results in a lot of e-mails.  This fix should stop that - someone should sanity check that it doesn't break anything before we pull it to all of our live sites.

Includes commit: Switch to new format for settings.CACHES
This should prevent warnings from being displayed whenever the code is
imported.
(cherry picked from commit 5829d326fca496914c73c093f765482d3578acd6)