Refactor form submission handling for sign up

- Disable submit button and display spinner during form submission to
prevent multiple clicks
- Obtain form field values and send signup request as form data for
processing on the server

static/build/index.js

@@ -104,8 +104,8 @@ function submitSignup() {
             throw new Error("Signup request failed");
         }
     })
-    .then(data => {
-        console.log(data);
+    .then(() => {
+        window.location.href = "/welcome";
     })
     .catch(error => {
         console.error(error);