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/time-element.html

@@ -0,0 +1,3 @@
+<time datetime="{{ .Format "2006-01-02T15:04:05Z07:00" }}" title="{{ . }}">
+    {{ .Format "January 2, 2006" }}
+</time>