refactor: migrate ENABLE_HTML_XBLOCK_STUDENT_VIEW_DATA off FEATURES-as-dict

Switch the built-in HtmlBlock reader from settings.FEATURES.get(...) to
getattr(settings, self.ENABLE_HTML_XBLOCK_STUDENT_VIEW_DATA, False) (the
flag is LMS-only, read from shared xmodule code), and convert the test
overrides to flat @override_settings.

The user-facing 'To enable, set FEATURES[...]' message is intentionally
left unchanged: the extracted xblocks_contrib.HtmlBlock (exercised by the
same tests via USE_EXTRACTED_HTML_BLOCK) still emits it, and the shared
assertion must match both. Flat override_settings still reaches the
extracted block's FEATURES.get reader through the FeaturesProxy bridge.