fix(DB/Creature): Hide various dungeon creature triggers (#1924)

Adding Extra_flags 128 (CREATURE_FLAG_EXTRA_TRIGGER) = Creature is trigger-NPC (invisible to players only)

- Shirrak the Dead Watcher in Heroic
- Zul'Aman - Jan'alai (Dragonhawk boss): Firebomb (Zul'Aman) 23920
- Mount Hyjal - Archimonde: Doomfire  18095

data/sql/updates/pending_db_world/rev_1559394390864887200.sql

@@ -0,0 +1,3 @@
+INSERT INTO `version_db_world` (`sql_rev`) VALUES ('1559394390864887200');
+
+UPDATE `creature_template` SET `flags_extra` = `flags_extra` | 128 WHERE `entry` IN (20308,23920,18095);