[BUGFIX] Also prefill datetime fields on US systems when going back from confirmation to form

Playing around helped me finding out that a "T" is missing if the OS is set to US language.

Author: einpraegsam

Resources/Private/Build/JavaScript/Form.js

@@ -180,7 +180,7 @@ class PowermailForm {
         if (type === 'date' || type === 'datetime-local' || type === 'time') {
           let formatOutput = 'YYYY-MM-DD';
           if (type === 'datetime-local') {
-            formatOutput = 'YYYY-MM-DD HH:mm';
+            formatOutput = 'YYYY-MM-DD\THH:mm';
           } else if (type === 'time') {
             formatOutput = 'HH:mm';
           }