Fixed #17919 - correct the behavior of the checkout type selector

[uberbrady]

We had a problem where, if you had checked an asset out to a non-User, when you would go back to checking out another asset, it would correctly highlight the last 'type' of thing you checked out and would actually show the correct drop-down menu, but it would not select the correct invisible radio button to state what the correct checkout_to_type was.

Additionally, when you would go to the checkout screen on a fresh session, no type of checkout target was selected. Whereas we had used to default to showing the User dropdown.

This fixes both of those issues.

It was really tempting (and I wrote it at first this way) to do some kind of @php...@endphp thing to determine $active_checkout_type and then take action on that to prevent us from repeating session('checkout_to_type') or session('checkout_to_type') ?? 'user' as often as we do, but that seemed like just extra noise and we tend to actively avoid @php sections, so I went with this simpler approach.

Fixes #17919