Separate functional settings from Theme settings.

[fabiocaccamo]

This is just a very-low priority idea at the moment, but maybe in the future it would become necessary.

Steps:

• Create a new Options model with the functional fields that are now in the Theme model.
• Automatically create an Option model instance if it doesn't exist.
• Allow only model changes, not add/delete permissions.
• Migrate options values from active Theme model to Options model (add also a command for running it ?!).
• Add template tag for loading options.
• Add caching.