Website: use SPF for the SPF site, part 2 - enable transitions.

- Build vendor assets into versioned directories.
- Use timestamped URLs for other JS/CSS.
- Give script/style tags names to avoid unnecessary reloads.
- Update main JS to initialize/dispose SPF.
- Add `spf-link` to the `html` tag to enable all links for SPF.
- Improve parsing when creating SPF JSON files in the Jekyll plugin.

Progress on #224.