Add FILE_UPLOAD_ENABLED setting (default to False, to avoid
a breaking change). Also adds a max_count_by_user sub setting
to restrict the number of uploaded files per user.
Refactor feature flag mechanism from Django permission classes to custom
decorator that returns 404 Not Found when features are disabled instead
of exposing API structure through permission errors.
Improves security by preventing information disclosure about disabled
features and provides more appropriate response semantics. Custom
decorator approach is better suited for feature toggling than Django's
permission system which is designed for authorization.
Florent Chehab