Replace Achievements with Advancements

- Advancements are now in JSON files, converted by the included migration script.
- WizardryAdvancementTrigger is a generic achievement trigger that can be reused throughout the code to trigger custom advancements not covered by the JSON conditions.
- Plus some other small and obvious fixes.

src/main/java/electroblob/wizardry/item/ItemSpellBook.java

@@ -25,6 +25,8 @@ import net.minecraftforge.fml.relauncher.Side;
 import net.minecraftforge.fml.relauncher.SideOnly;
 import net.minecraftforge.oredict.OreDictionary;
 
+import javax.annotation.Nullable;
+
 public class ItemSpellBook extends Item {
 
 	public ItemSpellBook(){