Accessibility improvements to the checkout links and empty header row

[kimcoleman]

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Changes proposed in this Pull Request:

Updated the links to checkout to include a more descriptive link aria-label like core PMPro.

Added screen reader text to empty header cell.

Changelog entry

• ENHANCEMENT: Improved accessibility of levels page buttons that link to checkout.
• ENHANCEMENT: Improved accessibility of table layout levels page.

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.