Disable form on admin appearance sign-in page preview

What does this MR do and why?

Fixes #24238 (closed)

The login form on the Admin Appearance preview page was previously active. While a security issue was fixed by !63895 (merged). I think this change is still useful in further reducing confusion by disabling the form completely. This will make it more clear that this is just a preview and is not the sign-in form.

Screenshots or screen recordings

On hover, title popover says the form is preview only and that's why it's disabled:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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