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/partials/footer.html

@@ -12,11 +12,11 @@
   </div>
 </footer>
 
-<script src="{{ "assets/main.js" | absURL }}"></script>
-<script src="{{ "assets/prism.js" | absURL }}"></script>
+<script defer src="{{ "assets/main.js" | relURL }}"></script>
+<script defer src="{{ "assets/prism.js" | relURL }}"></script>
 
 {{ if $.Site.Params.showLanguageSelector }}
-  <script src="{{ "assets/languageSelector.js" | absURL }}"></script>
+  <script src="{{ "assets/languageSelector.js" | relURL }}"></script>
 {{ end }}
 
 <!-- Extended footer section-->