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/classList.js

@@ -0,0 +1,20 @@
+/**
+ * The `classList` utility creates a list of class names by joining an object's
+ * keys, but only for values which are truthy.
+ *
+ * @example
+ * classList({ foo: true, bar: false, qux: 'qaz' });
+ * // "foo qux"
+ *
+ * @param {Object} classes
+ * @return {String}
+ */
+export default function classList(classes) {
+  const classNames = [];
+
+  for (const i in classes) {
+    if (classes[i]) classNames.push(i);
+  }
+
+  return classNames.join(' ');
+}