Added Validation checks for invalid kits - easier for server owners and developers to see the problem. Look out for "Kit must not be null".
Added The ability to not have the following messages sent (set them to "none" in language.yml):
- No Inventory Space
- Kit Redeem Message
- Kit Redeem Title
- Kit Redeem Subtitle
- Kit UpgradedMessage
Added Kit upgraded message caching
Fixed KitGiveEvent getting called even if the kit level is invalid