Site users should error when all users are removed

[allanaaa]

When you remove the last user from a site, it appears to save but just reverts back to how it was just before. (Says it saved successfully.) We should probably have a "You cannot remove all users from a site" or "A site cannot have zero users" error message there.

Alternatively, we could just allow removing all users from a site, knowing the Global Admin can add people back later.