Add platform-specific feature flag (TNL-659)

Student Notes: This is a platform-specific feature flag, NOT a course-specific advanced setting.

Issue: https://openedx.atlassian.net/browse/TNL-659

@polesye please look.
```python
is_studio = getattr(self.system, 'is_author_mode', False)
# edXNotes platform-specific feature flag.
feature_enabled = settings.FEATURES.get('ENABLE_EDXNOTES')

# edXNotes must be disabled in Studio or if feature is disabled.
if is_studio or not feature_enabled:
    return original_get_html(self, *args, **kwargs)
else:
    ...
```
I do not like duplication of `return original_get_html(self, *args, **kwargs)`. Can we use example above?