Add ON_SCRIPTS setting and use it

[adehnert]

The UA student/faculty dinners app has an ON_SCRIPTS setting that enables various things, perhaps chief among them the scripts-specific media URL. Given that local_settings_after.py generally contains *only* the scripts-specific media URL on most of our scripts deployments, we should add the ON_SCRIPTS setting, use it for the media URLs, and possibly kill off local_settings_after (or at least empty it on most systems).