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/_shortcodes/abbr.html

@@ -0,0 +1,5 @@
+{{ if .IsNamedParams }}
+    <abbr title="{{ .Get "title" }}">{{ .Inner }}</abbr>
+{{ else }}
+    <abbr title="{{ .Get 0 }}">{{ .Inner }}</abbr>
+{{ end }}