fix: Fixes spell mechanics and misc bugs (#1118)

- [X] Fixes NPE from account tags.
- [X] Fixes bad skill check due to missing cast to double.
- [X] Fixes water elemental duration.
- [X] Standardizes spell summon duration by expansion.

Projects/UOContent/Spells/Ninjitsu/MirrorImage.cs

@@ -170,7 +170,7 @@ namespace Server.Mobiles
 
             var duration = TimeSpan.FromSeconds(30 + caster.Skills.Ninjitsu.Fixed / 40);
 
-            new UnsummonTimer(caster, this, duration).Start();
+            new UnsummonTimer(this, duration).Start();
             SummonEnd = Core.Now + duration;
 
             MirrorImage.AddClone(m_Caster);