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

_includes/base.njk

@@ -25,6 +25,7 @@
     <nav class="footer-nav">
       <a href="/about/">about</a>
       <a href="/code/">code</a>
+      <a href="/subjects/">subjects</a>
     </nav>
   </footer>
 </body>

_includes/column.njk

@@ -38,6 +38,7 @@
     <nav class="footer-nav">
       <a href="/about/">about</a>
       <a href="/code/">code</a>
+      <a href="/subjects/">subjects</a>
     </nav>
   </footer>
 </body>