Add custom 404 page, and other things

- Create custom 404 page
- Modify index.php to include custom 404 page when querying a
nonexistent resource
- Increase spacing between <li> elements
- Modify privacy policy, because it turns out CloudFlare uses cookies

New files:
- 404.php, a custom 404 page

Potential issues:
- 404 page only currently works when attempting to load a resource
following the format "dendodge.me?p=foo"; .htaccess modifications could
be used to apply it to all non-existent pages, but this will do for now