When a card has a single target, allow one tap retargeting

[DylanRJohnston]

The card targeting screen is built around supporting multi-target cards, i.e. you select up to n targets, to select a new target you must deselect one first. This is natural for N > 1, but when the card has a single target, the expectation is that clicking a new target automatically deselects the old one, this also affects the visual "disabled" state when the choice is fully saturated i.e. when a card has a single target, all other valid targets should remain "active" instead of greying out like they do in the N > 1 case.