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/forum/src/components/HeaderPrimary.js

@@ -0,0 +1,26 @@
+import Component from 'flarum/Component';
+import ItemList from 'flarum/utils/ItemList';
+import listItems from 'flarum/helpers/listItems';
+
+/**
+ * The `HeaderPrimary` component displays primary header controls. On the
+ * default skin, these are shown just to the right of the forum title.
+ */
+export default class HeaderPrimary extends Component {
+  view() {
+    return (
+      <ul className="header-controls">
+        {listItems(this.items().toArray())}
+      </ul>
+    );
+  }
+
+  /**
+   * Build an item list for the controls.
+   *
+   * @return {ItemList}
+   */
+  items() {
+    return new ItemList();
+  }
+}