Add support for custom Hopper inventories. Fixes BUKKIT-4722

Opening a hopper inventory created by Server.createInventory will
currently have no effect as proper handling code is missing in
CraftEntityHuman for hopper inventories that aren't associated with a tile
entity or minecart. Initialization logic for hoppers is also missing from
CraftContainer, which is necessary for the opening of custom hopper
inventories.

This commit fixes the two aforementioned by adding proper handling to
CraftHumanEntity for opening inventories not associated with a tile
entity, and by adding initialization logic for hoppers to CraftContainer.