Numerous performance fixes.

- Refactored themes to always load with just a single file. This avoids
  a round trip on each request.
- Locally load "Fira Code" when available on the system, then try to
  load the woff2 file, then fall back to regular woff.
- Update markup to use relative urls where it is safe. RSS feeds still
  use absolute URLs.
- Only set og:image:(height/width) when using the standard color icon.
- Fixed styles on the pagination buttons.

layouts/_default/terms.html

@@ -8,7 +8,7 @@
       {{ $count := .Count }}
       {{ with $.Site.GetPage (printf "/%s/%s" $type $name) }}
         <li>
-          <a class="terms-title" href="{{ .Permalink }}">{{ .Name }} ({{ $count }})</a>
+          <a class="terms-title" href="{{ .RelPermalink }}">{{ .Name }} ({{ $count }})</a>
         </li>
       {{ end }}
       {{ end }}