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/FieldSet.js

@@ -0,0 +1,22 @@
+import Component from 'flarum/Component';
+import listItems from 'flarum/helpers/listItems';
+
+/**
+ * The `FieldSet` component defines a collection of fields, displayed in a list
+ * underneath a title. Accepted properties are:
+ *
+ * - `className` The class name for the fieldset.
+ * - `label` The title of this group of fields.
+ *
+ * The children should be an array of items to show in the fieldset.
+ */
+export default class FieldSet extends Component {
+  view() {
+    return (
+      <fieldset className={this.props.className}>
+        <legend>{this.props.label}</legend>
+        <ul>{listItems(this.props.children)}</ul>
+      </fieldset>
+    );
+  }
+}