Implement "password"-based session joining.

src/Go/views/game.ecr

@@ -9,6 +9,9 @@
     .black-cell {
         background-color: black;
     }
+    .white-cell {
+        background-color: white;
+    }
     .board {
         max-width: 360px;
     }
@@ -22,7 +25,7 @@
         var app = Elm.Main.embed(node, {
             'black' : <%= black %>,
             'url' : "<%= "ws://" + URL + ":" + Kemal.config.port.to_s %>",
-            'id' : <%= id %>,
+            'id' : "<%= id %>",
             'size' : <%= size %>
         })
     </script>