Replace Potions of Confusion with Ambrosia

A new potion, inspired by the old food item. Causes confusion
and an Elixir-like HP & MP-regeneration effect. If the confusion
is cured or prevented, the regeneration goes away.

[Committer's note: tweaked a bunch of stuff.]
Restore Spider's sweet ambrosia

This reverts commit 24381c416777577198129986a9082a3d73d2224d.