Factor out the tree rendering

src/root/static/css/tree.css

@@ -0,0 +1,44 @@
+ul.tree, ul.subtree {
+    padding: 0;
+    margin: 0;
+    list-style-type: none;
+}
+
+/* Indent each tree level. */
+li.tree-line {
+    position: relative;
+    padding-left: 2.0em;
+}
+
+li.tree-line:last-child {
+    padding-left: 2.1em; /* for the 0.1em border-left in the last child */
+}
+
+/* Draw vertical tree lines next to all children except the last. */
+li.tree-line {
+    border-left: 0.1em solid #6185a0;
+}
+
+li.tree-line:last-child {
+    border-left: 0;
+}
+
+/* Draw horizontal tree lines to all children. */
+span.tree-conn {
+    position: absolute;
+    height: 0.65em;
+    left: 0em;
+    width: 1.5em;
+    border-bottom: 0.1em solid #6185a0;
+}
+
+/* Draw the vertical tree line to the last child. */
+li.tree-line:last-child > span.tree-conn {
+    border-left: 0.1em solid #6185a0;
+}
+
+.tree-toggle {
+    text-decoration: none;
+    font-family: monospace;
+    font-size: larger;
+}