Major overhaul of the architecture

It's pretty horrible to have a commit messing up 90% of the code and
also stuff isn't really well organized, but probably it needed to be
done...

frontend/index.html

@@ -5,6 +5,6 @@
   <title>Chess</title>
 </head>
 <body>
-  <script src="main.js"></script>
+  <script type="module" src="main.js"></script>
 </body>
 </html>