Massive JavaScript cleanup

- Use JSX for templates
- Docblock/comment everything
- Mostly passes ESLint (still some work to do)
- Lots of renaming, refactoring, etc.

CSS hasn't been updated yet.

js/lib/utils/ItemList.js

@@ -0,0 +1,61 @@
+class Item {
+  constructor(content, priority) {
+    this.content = content;
+    this.priority = priority;
+  }
+}
+
+/**
+ * The `ItemList` class collects items and then arranges them into an array
+ * by priority.
+ */
+export default class ItemList {
+  /**
+   * Add an item to the list.
+   *
+   * @param {String} key A unique key for the item.
+   * @param {*} content The item's content.
+   * @param {Integer} [priority] The priority of the item. Items with a higher
+   *     priority will be positioned before items with a lower priority.
+   * @public
+   */
+  add(key, content, priority) {
+    this[key] = new Item(content, priority);
+  }
+
+  /**
+   * Merge another list's items into this one.
+   *
+   * @param {ItemList} items
+   * @public
+   */
+  merge(items) {
+    for (const i in items) {
+      if (items.hasOwnProperty(i) && items[i] instanceof Item) {
+        this[i] = items[i];
+      }
+    }
+  }
+
+  /**
+   * Convert the list into an array of item content arranged by priority. Each
+   * item's content will be assigned an `itemName` property equal to the item's
+   * unique key.
+   *
+   * @return {Array}
+   * @public
+   */
+  toArray() {
+    const items = [];
+
+    for (const i in this) {
+      if (this.hasOwnProperty(i) && this[i] instanceof Item) {
+        this[i].content.itemName = i;
+        items.push(this[i]);
+      }
+    }
+
+    return items.sort((a, b) => b.priority - a.priority).map(item => item.content);
+  }
+}
+