[Experience] Add exp mod to npc types to let a server op change the exp modifier (#1252)

* Add exp mod to npc types to let a server op change the exp modifier a npc gives (useful for custom content)

* Updated version.h

utils/sql/git/required/2021_02_14_npc_exp_mod.sql

@@ -0,0 +1 @@
+ALTER TABLE `npc_types` ADD COLUMN `exp_mod` INT NOT NULL DEFAULT '100' AFTER `always_aggro`;