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/templates/duoSignedResponseField.html

@@ -1,3 +1,4 @@
 <div class="duo-signature-response-field" ng-class="{ loading : !duoInterfaceLoaded }">
     <iframe></iframe>
+    <input type="submit">
 </div>