Reinstate "Move notes.scss to page_bundles from page_specific_bundles"

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Description

!131241 (merged) had to be reverted (see revert) because it broke dark mode on .com. It appears that the FF on vs. off did not affect this.

I'm thinking this is because of the heavy use of scss variables instead of css variables in the notes.scss file.

TODO

• Revisit !131241 (merged) and open a new MR that fixes this regression
• Re-enable page_specific_styles feature flag on .com (I disabled it while trying to resolve gitlab-com/gl-infra/production#17209 (closed).