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/mixin.js

@@ -1,11 +1,20 @@
+/**
+ * The `mixin` utility assigns the properties of a set of 'mixin' objects to
+ * the prototype of a parent object.
+ *
+ * @example
+ * class MyClass extends mixin(ExtistingClass, evented, etc) {}
+ *
+ * @param {Class} Parent The class to extend the new class from.
+ * @param {...Object} mixins The objects to mix in.
+ * @return {Class} A new class that extends Parent and contains the mixins.
+ */
 export default function mixin(Parent, ...mixins) {
   class Mixed extends Parent {}
-  for (var i in mixins) {
-    var keys = Object.keys(mixins[i]);
-    for (var j in keys) {
-      var prop = keys[j];
-      Mixed.prototype[prop] = mixins[i][prop];
-    }
-  }
+
+  mixins.forEach(object => {
+    Object.assign(Mixed.prototype, object);
+  });
+
   return Mixed;
 }