use `solution-placement="{right|bottom|none}"` rather than `format=` attribute

[armandofox]

in the existing PL element pl-order-blocks, it uses solution-placement="right|bottom" to specify where the target tray goes. we should use this same syntax but also allowing solution-placement="none" to indicate when only a single tray should appear.
we should also leave the current format= so that legacy questions don't break - ie make format an alias for solution-placement that we can deprecate later.