Move the addition of the FIT tag out of the item constructor.

It is now applied there:
- npc starting inventory and worn items (1/3 chance)
- result of crafting (always), including byproducts
- spawning items on the map (e.g. mapgen) (1/3 chance)
- spawning items through item groups (1/3 chance)

Adding the tag does not happen in finalize_crafted_item, but in recipe::create_result instead. That way the item used to display the stats of the result also has the FIT tag.