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/cover.html

@@ -5,7 +5,7 @@
   {{- if .Resources.GetMatch .Params.Cover }}
     {{- $cover = (.Resources.GetMatch .Params.Cover).RelPermalink -}}
   {{- else -}}
-    {{- $cover = absURL .Params.Cover -}}
+    {{- $cover = relURL .Params.Cover -}}
   {{- end -}}
 {{- else if $.Site.Params.AutoCover -}}
   {{- if (not .Params.Cover) -}}