Restore previous formation behaviour of targeting nearby enemies when idle / aborting orders
This restores A23 formation behaviour, changed by rP22447. Units will now start attacking nearby enemies when the formation goes idle (such as when the abort order button is clicked).
This introduces a change that formation units that actually go idle will try to move back in formation shape, so it looks more like they're still in formation.
Fixes #5519.
Differential Revision: https://code.wildfiregames.com/D2097