Accessibility improvements and bugfixes

Highlights

• Search improvements
• Analytics improvements (Google Analytics and Plausible)

Search improvements

• Improve search form accessibility
• Allow search feature to work on static websites loaded from filesystem (and not a webserver) #198

Analytics changes

• Fix Google Analytics always being added to the page #194
• Use latest Google Analytics tracking code #202
• Add https://plausible.io analytics

Other changes

• Update to CommonMark 1.4, use embedded table extension
• Add rel="noopener noreferrer" on external links