Django settings cleanup

I removed all settings referring to Django < 1.4, added some comments and unified some things. There are still two settings files, but I renamed `settings_apache` to `settings_production`.  Also the input file for the configuration generator changed a bit.

To update CATMAID instances to this, it would be best to create a new `configuration.py` file and recreate the Django settings files with `create_configuration.py`.