Refactor internal default settings

[vsalvino]

This effectively fixes #8

• Refactored internal settings to behave more like Django settings
• Use this opportunity to rename stuff to crx_* for consistency.
• Lazy load any settings being used as choices in concrete model fields. This will totally eliminate the issue of sites generating migrations back in coderedcms. It's a bit of a hack (that we were already employing elsewhere) but will be a good quality of life improvement until Make built-in models abstract #56 is completely implemented and we have no more concrete models (which may never even happen)