Organize columns by subject tags; add /subjects/ page

- Drive the columns collection from the `column` tag, recovering 3 columns
  authored under Notes/ that the glob missed
- Tag the 6 columns with subject tags; add a subjects collection and a
  /subjects/ listing page (newest-first), linked from the footer

subjects.njk

@@ -0,0 +1,23 @@
+---
+layout: base.njk
+title: Subjects
+permalink: /subjects/
+---
+
+<h1>Subjects</h1>
+
+{%- for subject in collections.subjects %}
+<section class="subject-group">
+  <h2>{{ subject }}</h2>
+  <ul>
+    {%- for post in collections[subject] | sort(true, false, "date") %}
+    <li>
+      <a href="{{ post.url }}">{{ post.data.title | inlineMarkdown | safe }}</a>
+      <time datetime="{{ post.date | isoDate }}">{{ post.date | readableDate }}</time>
+    </li>
+    {%- endfor %}
+  </ul>
+</section>
+{%- else %}
+<p>No subjects yet.</p>
+{%- endfor %}