Clean up CSS

- Add vim folds to make organized sections
- Rename a lot of classes from post- to page-
- Generalize series CSS classes better
- Clean up navbar div structure
- Remove langpicker and most of multilingual support
- many other minor tweaks

layouts/partials/page-metadata/item/series.html

@@ -0,0 +1,11 @@
+{{ with (.GetTerms "series") }}
+    {{ $series := slice }}
+    {{ range . }}
+        {{ $series = $series | append (printf `<a href="%s">%s</a>` .Permalink .LinkTitle) }}
+    {{ end }}
+
+    <div class="page-metadata-section series" aria-label="{{ i18n "aria_post_meta_series" }}">
+        {{ partial "icon.html" "files" }}
+        {{ delimit $series ", " }}
+    </div>
+{{ end }}