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/404.html

@@ -3,7 +3,7 @@
     <h1 class="post-title">404 — {{ $.Site.Params.missingContentMessage | default "Page not found..." }}</h1>
 
     <div class="post-content">
-      <a href="{{ "/" | absURL }}">{{ $.Site.Params.missingBackButtonLabel | default "Back to home page" }}&nbsp;→</a>
+      <a href="{{ "/" | relURL }}">{{ $.Site.Params.missingBackButtonLabel | default "Back to home page" }}&nbsp;→</a>
     </div>
 
   </div>