Make the settings more secure and generic
At the moment, there are several issues with the setting file:
- some settings shouldn't be versionned (SECRET_KEY)
- some settings must be duplicated in local_settings to be overriden (DB, DEBUG, STATIC_URL, PADS_*…)
- local_settings.py can't read from settings which doesn't allow for if conditions based on settings.py settings (like enable Django Debug Toolbar based on the value of DEBUG but it may be not the best example)