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/list.html

@@ -8,7 +8,7 @@
     {{ range .Paginator.Pages }}
       <div class="post on-list">
         <h1 class="post-title">
-          <a href="{{ .Permalink }}">{{ .Title | markdownify }}</a>
+          <a href="{{ .RelPermalink }}">{{ .Title | markdownify }}</a>
         </h1>
         <div class="post-meta">
           <span class="post-date">
@@ -22,7 +22,7 @@
         {{ if .Params.tags }}
           <span class="post-tags">
             {{ range .Params.tags }}
-            #<a href="{{ (urlize (printf "tags/%s/" . )) | absLangURL }}">
+            #<a href="{{ (urlize (printf "tags/%s/" . )) | relLangURL }}">
               {{- . -}}
             </a>&nbsp;
             {{ end }}