[Cleanup] Mask GM Show Buff message behind EntityVariable (#4419)

* [Cleanup] Mask GM Show Buff message behind EntityVariable

Removes the spam of "Your GM flag allows you to always see your targets' buffs." for GMs every time a buff lands on a target.

It will now lock to an Entity Variable and only show once per zone.

* Convert string to constexpr

* Switch from string to char

zone/entity.h

@@ -60,6 +60,8 @@ class Bot;
 
 extern EntityList entity_list;
 
+constexpr const char* SEE_BUFFS_FLAG = "see_buffs_flag";
+
 class Entity
 {
 public: