generate subpage listings automatically

right now very barebones!

- doesn't sort pages quite correctly
- no search function
- still not sure about the UI design aspects

includes refactor of tree generation code

static/js/tree.js

@@ -73,9 +73,14 @@ export class Branch {
             });
         }
 
-        if (ulid.isCanonicalUlid(this.namedID)) {
-            // Adjust dates to fit the user's time zone.
-            let timestamp = ulid.getTimestamp(this.namedID);
+        // Adjust dates to fit the user's time zone.
+        let timestamp = null;
+        if (element.hasAttribute("th-ts")) {
+            timestamp = new Date(parseInt(element.getAttribute("th-ts")));
+        } else if (ulid.isCanonicalUlid(this.namedID)) {
+            timestamp = ulid.getTimestamp(this.namedID);
+        }
+        if (timestamp != null) {
             let branchDate = this.buttonBar.getElementsByTagName("th-bd")[0];
             branchDate.textContent = dateToString(timestamp);
         }