Scripts/Ulduar: Spellscripts for Mimiron

Cutting the commit in pieces, so that the diff can be viewed on github...
Change this to GetHitCreature() && GetHitCreature()->IsSummon() - will make this slightly more obvious and will allow you to drop ToTempSummon() completely (replace GetHitUnit with GetHitCreature in all cases inside this method (HandleDespawn))