GUACAMOLE-136: Use hidden submit button for submitting Duo response (form.submit() inserts query parameters and causes browser navigation, despite ngSubmit preventing default).

extensions/guacamole-auth-duo/src/main/resources/styles/duo.css

@@ -17,6 +17,10 @@
  * under the License.
  */
 
+.duo-signature-response-field input[type="submit"] {
+    display: none !important;
+}
+
 .duo-signature-response-field iframe {
     width: 100%;
     max-width: 620px;