Move template files based on Hugo v0.146 revamp

See https://gohugo.io/templates/new-templatesystem-overview/

Under layouts/
- mv _default/* .
- mv partials _partials
- mv shortcodes _shortcodes
- mv taxonomy/list.html taxonomy.html
- mv taxonomy/term.html term.html

layouts/_partials/series-box.html

@@ -0,0 +1,17 @@
+{{ with (.GetTerms "series") }}
+    {{ $series := slice }}
+    {{ range . }}
+        {{ $series = $series | append (printf `<a href="%s">%s</a>` .Permalink .LinkTitle) }}
+    {{ end }}
+
+    <div class="series-box">
+        {{ partial "icon.html" "info" }}
+        {{ if gt (len $series) 1 }}
+            This post is part of multiple series:
+        {{ else }}
+            This post is part of a series:
+        {{ end }}
+
+        {{ delimit $series ", " | safeHTML }}
+    </div>
+{{ end }}