UCLDC_DEBUG environment variable should turn off DEBUG if false

[amywieliczka]

Currently, in order to turn off Django's debug functionality, the UCLDC_DEBUG environment variable needs to be completely unset (https://github.com/cdlib/pad-rikolti-deploy/commit/fa628e336376ed07c710da07ae8e7807f340d510).

I've removed the UCLDC_DEBUG variable from the Cloudformation template, but that means for a -stage or -test instance, we need to manually add it.

Instead, we should modify the application to accept falsy values for UCLDC_DEBUG as flags to turn off Django's debug, then we should add the UCLDC_DEBUG environment variable back into the Cloudformation template, and set it to true for -stage and false for production beanstalks