Clean up hidden accessibility text class

Closes #4662

- Replace the `.sr-only` class with `.hidden` for readability and consistency
- Remove `.sr-only-focusable` as it wasn't used anywhere
- Clean up `.hidden` class properties to be better for screen readers

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Note: There are docs and temporary HTML files which still contain these old classes, but they're in dire need of love or deleting, so I've not touched them.