Core/Loot: Skinning improvements

* Set skinnable flag only after all loot was taken
* Creatures are skinnable only once - after skinning, the player only has to loot the creature again if he did not take all skinning loot
Scripts/MagistersTerrace: reworked InstanceScript and fixed some issues on boss Vexallus
* removed old hacks and added missing auras
* Vexallus converted to EventMap
Merge pull request #12364 from MitchesD/misc_2

Scripts/MagistersTerrace: reworked InstanceScript and fixed some issues ...
Core: Fix warnings and kill again whitespace ...
Core/Mail: Fix exploit that allowed to send free emails
Script/MagistersTerrace: Fix fail in 27f28552489bab153d5c4717ce48e31496b4c43f
Merge branch 'master' of https://github.com/TrinityCore/TrinityCore into ghbb
Core/Misc: Define CREATURE_TYPEFLAGS_PROJECTILE_COLLISION
Core/Spells: Ensure Haunt's healing takes in consideration crit damage
Merge branch 'master' of https://github.com/TrinityCore/TrinityCore into ghbb
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