core: hide spinner for initial install

core/js/setup.js has logic to show the spinner upon form submission, but
ever since v12 was released the spinner was never hidden in the first
place.

Modify core/css/guest.css to hide it, which allows core/js/setup.js to
do its thing.

Fix #5532

Signed-off-by: Kyle Fazzari <kyrofa@ubuntu.com>