Various suggested changes in separate commits

[jeroenheijmans]

Adding four things, each in its separate commit if you'd prefer to cherry-pick only a subset of them:

1. A custom favicon instead of the default vercel one
2. QR code for larger viewports, so folks could scan it to go the the puzzle-picker when the website is shown on a projector or large screen at a meetup
3. Responsiveness improvements
4. Filter puzzles based on years (for those of us that have already completed certain years)

Here's a demo: