Use app-based settings

[benjaoming]

We need to complete the last part of #3757 - details are in this Issue about how to work with project settings vs. app settings and re-introduce the possibility of more convenient "derived" settings where sensible.

This is somewhat necessary to finally get rid of local_settings when we consider a setting like CONTENT_ROOT that could potentially be a very efficient way of changing lots of other settings. Currently, they would all have to be specified manually in ~/.kalite/settings.py which is ugly.