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/readingtime.html

@@ -0,0 +1,6 @@
+{{ if ne .Site.Params.readingtime false }}
+    <span class="page-metadata-item" aria-label="{{ i18n "aria_post_meta_reading_time" }}">
+        {{ partial "icon.html" "clock" }}
+        {{ i18n "readingtime" .ReadingTime }}
+    </span>
+{{ end }}