DIsable submit button when clicked

static/index.html.tmpl

@@ -66,6 +66,12 @@
             border-radius: 5px;
             cursor: pointer;
         }
+         
+        #submit-btn:disabled {
+            background-color: #ccc;
+            color: #666;
+            cursor: not-allowed;
+        }
 
         #submit-btn:hover {
             background-color: #45a049;
@@ -166,6 +172,8 @@
                 return;
             }
 
+            submitBtn.disabled = true;
+
             const days = calendar.children;
             const choices = Array.from(days).map(day =>
                 Array.from(day.children)