We want to use SIWEPanel in the new registration flow, but it currently renders a large spinner with flex: 1 while loading, which doesn't match the designs for the new registration flow.

To make SIWEPanel usable in both the old and new context, this diff factors out the spinner code into FullscreenSIWEPanel, making SIWEPanel more reusable.

Depends on D7840

Make sure the loading spinner still appears full-screen in the old experience