Accessibility review - pt 2

[AndrewRoberts1]

Issues

• Empty table header on table checkbox comp when there are no checkboxes.
• Heading issue still present https://github.com/orgs/NaturalHistoryMuseum/projects/43/views/1?pane=issue&itemId=136984581&issue=NaturalHistoryMuseum%7Cjoin-the-dots-dev%7C54
• Add 'main' html tag for better screen nav