Fix semantic nav and asides on mobile + other improvements

[lkhrs]

Fixes #96

• Consolidate appearance reset
• better styling for progress bars
• Consolidate box styling
• Style fieldsets
• Add test page for preview deploys Replaces CDN landing
• Avoid overflow in header navigation Fixes Header, nav bar, and asides don't fit well on mobile #96
• Fix margin issues in header when using semantic HTML Fixes Header, nav bar, and asides don't fit well on mobile #96
• Reduce font size in asides, set to full-width on mobile Fixes Header, nav bar, and asides don't fit well on mobile #96

[netlify]

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