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/remote_figure.html

@@ -0,0 +1,28 @@
+{{ $hidecaption := index . "hidecaption" }}
+{{ $type := index . "type" }}
+{{ $src := index . "src" }}
+{{ $alt := index . "alt" }}
+
+{{ if index . "border" }}
+    <figure class="border">
+{{ else }}
+    <figure>
+{{ end }}
+
+    {{ if eq $type "image" }}
+        <img src="{{ $src }}" alt="{{ $alt }}" loading="lazy" />
+    {{ else if eq $type "video" }}
+        <video controls preload="metadata">
+            <source src="{{ $src }}" alt="{{ $alt }}">
+            {{ i18n "browser_no_video_support" }}
+        </video>
+    {{ end }}
+
+    {{ if not $hidecaption }}
+        {{ with $alt }}
+            <figcaption>
+                {{ . | markdownify }}
+            </figcaption>
+        {{ end }}
+    {{ end }}
+</figure>