Add google sign-in

Also, try to be a bit more correct with CSRF and CORS.
It works on my machine.

frontend/index.html

@@ -14,6 +14,7 @@
 
         <!-- Compiled and minified JavaScript -->
         <script src="https://cdnjs.cloudflare.com/ajax/libs/materialize/1.0.0/js/materialize.min.js"></script>
+        <script src="https://accounts.google.com/gsi/client"></script>
 
         <script src="jscolor.min.js"></script>
         <script src="config.js"></script>
@@ -46,6 +47,7 @@ main {
         </nav>
         <main class="container row">
             <p></p>
+            <div class="col s4 offset-s4" id="signin-prompt" style="display: none"></div>
             <div class="col m6 offset-m3 s12">
                 <a class="modal-trigger btn orange hide-on-large-only" href="#newproduct"><i class="material-icons left">add</i>Nouveau produit</a>
                 <ul class="collapsible" id="appbody">