changed the open sound to "ENTITY_HORSE_ARMOR" and made it customizable in settings.yml.
Here's a list of all minecraft sounds:
https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/Sound.html

Changelog:
- the command /backpackitems won't give you all backpacks anymore, but you have to specifiy which one you want
- slightly changed the chat message of /reloadbackpacks
- fixed the data saving issue with items that have a lot of data in them by saving it externally in a .yml file.
- the registered recipe isn't made with a player head anymore, but with the backpack item

When you pick up or craft an item required to craft a backpack (that isn't an upgrade of another backpack), the recipe is given to the player and displayed in the recipe book. When a backpack requires another backpack in the crafting grid, the recipe is unlocked once you craft that specific backpack. It wasn't as hard as I thought to implement.

+ fixed a small bug that it didn't work using a backpack in multiple recipes (now you can have multiple upgrade-backpacks for one backpack)