split locked into storeable, discardable, tradeable -- added Trade and Discard as Item options

The Locked component was preventing three different actions: Whether an item could be stored in the convoy Whether an item could be discarded outright Whether an item could be traded (to another unit or shop)

This splits these into individual item_system hooks and removes locked.

This MR also adds a constant to determine whether the player will have access to storing items into the convoy on the battlefield even when not near a battlefield convoy.

It also adds the Trade ability to the Item Options Menu.