refactor: migrate ENABLE_ENTERPRISE_INTEGRATION off FEATURES-as-dict

Switch the enterprise_enabled() reader to getattr(settings, ..., False)
(LMS-only flag read from openedx/features code reachable from CMS), and
convert all test overrides to flat @override_settings. This removes the
shared FEATURES_WITH_ENTERPRISE_ENABLED test helper (a settings.FEATURES
copy with the flag set) in favor of @override_settings(ENABLE_ENTERPRISE_INTEGRATION=True)
at each call site, and pulls the flag out of a DISABLE_START_DATES
patch.dict (that skip-list flag stays on the dict for now).