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/single.html

@@ -1,7 +1,7 @@
 {{ define "main" }}
 <div class="post">
   <h1 class="post-title">
-    <a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></h1>
+    <a href="{{ .RelPermalink }}">{{ .Title | markdownify }}</a></h1>
   <div class="post-meta">
     {{ if .Params.Date }}
       <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>&nbsp;
+    #<a href="{{ (urlize (printf "tags/%s/" .)) | relLangURL }}">{{ . }}</a>&nbsp;
     {{ end }}
   </span>
   {{ end }}