Core/Spells: Fix Ebon Plague and Wandering Plague

Ebon Plague fix by @Unholychick
Wandering Plague fix by @tobmaps
Thanks to @maxxx for Ebon Plague correction.
Core/ObjectAccessor: don't use singleton when calling static methods
Merge pull request #13334 from Exodius/master

Core/Spells: Fix Ebon Plague and Wandering Plague
Merge branch 'master' of https://github.com/TrinityCore/TrinityCore into master_revamp

Conflicts:
	src/server/game/Spells/SpellMgr.cpp