Added bag space stat and associated bag upgrade items

Implemented #718 (closed) with @Pfauenauge

• Added inv_slots field to Stats - defaults to 18 for new characters, all existing characters have 36 slots grandfathered in
• Added 4 new bag upgrade consumables, crafting recipes, and associated loot table changes
• Added /set_inv_slots <amount> admin command for testing - items in removed slots are dropped on the floor at the player's feet
• Added Effect::InventorySlotIncrease effect and modified apply_effect to return Result which is required when an inventory slot increase fails

• By opening this merge request, you agree to release your code and all other changes under the GPL 3 license and to abide by the terms set by this license.
• Migrations have been added if applicable
• Significant changes of this merge request have been added to the changelog.