refactor: migrate ENABLE_FINANCIAL_ASSISTANCE_FORM off FEATURES-as-dict

The flag had no envs definition and was only read via settings.FEATURES.get
(with lms/envs/test.py already setting the flat value, routed through the
FeaturesProxy bridge). Add an annotated flat DjangoSetting definition
(default False) to lms/envs/common.py and migrate the two LMS-only readers.
In course_tools.py this also drops the now-unnecessary 'FEATURES is None'
(Edge) guard, since the flat setting always resolves to its default.