Scripts/ScarletMonastery: Summon the Headless Horseman once the Call the Headless Horseman is quest complete

By @cr4st
Closes #13273
Merge branch '3.3.5' of https://github.com/TrinityCore/TrinityCore into ghbb

Conflicts:
	src/server/scripts/EasternKingdoms/ScarletMonastery/boss_headless_horseman.cpp
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