Use the new KeyboardEvent instead of deprecated KeyEvent and
initKeyEvent, see
https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/initKeyEvent#specifications
and
slapos!800 (comment 115488)

In old firefox dispatching a fake keydown event with keycode 13 (enter)
on an input field was enough to submit the corresponding form, like in
a real browser and like in webdriver, but in modern browsers this no
longer submits the form. Adjust the logic to submit the form to mimic
real browser behavior.
Co-authored-by: Romain Courteaud <romain@nexedi.com>