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/shortcodes/code.html

@@ -11,5 +11,5 @@
     <pre {{ if .Get "language" }}class="language-{{ .Get "language" }}" {{ end }}><code>{{ .Inner | string }}</code></pre>
   </div>
 {{ else }}
-  {{ errorf "If you want to use the \"collapsable code\" shortcode, you need to pass a mandatory \"language\" param. The issue occured in %q (%q)" .Page.File .Page.Permalink }}
+  {{ errorf "If you want to use the \"collapsable code\" shortcode, you need to pass a mandatory \"language\" param. The issue occured in %q (%q)" .Page.File .Page.RelPermalink }}
 {{ end }}