Minor UX: Add spinner/animation/state change to Import button

[stevejalim]

Minor thing, but after a Transfer has been configured on /admin/wagtail-transfer/choose/ and the user hits 'Import XXX', the button is not put into a disabled/unclickable state, and the only indication that import is happening is the browser's own loading spinner. This means users can end up triggering a second import "just in case I didn't click on it", which can tie up resources and take longer.