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/components/SelectDropdown.js

@@ -0,0 +1,25 @@
+import Dropdown from 'flarum/components/Dropdown';
+import icon from 'flarum/helpers/icon';
+
+/**
+ * The `SelectDropdown` component is the same as a `Dropdown`, except the toggle
+ * button's label is set as the label of the first child which has a truthy
+ * `active` prop.
+ */
+export default class SelectDropdown extends Dropdown {
+  static initProps(props) {
+    super.initProps(props);
+
+    props.className += ' select-dropdown';
+  }
+
+  getButtonContent() {
+    const activeChild = this.props.children.filter(child => child.props.active)[0];
+    const label = activeChild && activeChild.props.label;
+
+    return [
+      <span className="label">{label}</span>,
+      icon('sort', {className: 'caret'})
+    ];
+  }
+}