Strike through on overriden features in real time

In item editor, when a feature that overrides a product feature (which is visible in the same page) is inserted, add a strike through (i.e. some <del>s or a class + ARIA attributes) to the product feature.

When the feature is removed, remove the strike through.

This also needs to be done server-side, when generating the editor page.

This is currently done server-side only for the "item viewer" page, not for the editor.