Up to now local_settings.py was used for binder specific configuration values.
This commit moves these values to settings.py, removes local_settings.py
from version control and adds logic to settings.py to allow settings in
local_settings.py to overwrite ones defined in settings.py, enabling
local_settings.py to be used as installation specific configuration file,
e.g. for custom database configuration.
To quote from Djangos documentation:
"The secret key must be a large random value and it must be kept secret."
https://docs.djangoproject.com/en/1.7/howto/deployment/checklist/#secret-key
This commit achieves that by generating a file containing a random string
when running the first time and by using this string as SECRET_KEY from then
on.