[Core] Improve handling of SPELL_AURA_EMPATHY
This will
* Allow use of Empathy on players (currently incorrect information might
* still be displayed)
* Fix Empathy such that only the player who cast empathy will get the
* special information
* Remove Empathy Auras when shapeshifting

Credits to virusav (c12797)
[Core] Implement SMSG_MOVE_SET_COLLISION_HGT to properly handle model
height changes for mounted/unmounted players.

Credits to Machiavell1 (c12798)
[Core] Fix build after last couple of commits