Refactor head partial

Switch commented-out resources to conditional resources. Now these CSS
and JS files can be sourced on a per-page basis. Simply set e.g.
`lightbox = true` in the header of a Markdown file.

Removed outdated `navi.css`.

Added helpful comments and reorganized resources by type rather than
source.

Added logic to JS inline script so that the list navigation can be
tabbed through and the focused item will trigger on hitting Enter.

Initial commit