Implement group mentor, leadership exp sharing (SoF+ only)

Currently only works in normal groups
Some decisions: the EXP will be rounded in the favor of the group leader
No idea how live actually handles it.

common/emu_oplist.h

@@ -207,6 +207,7 @@ N(OP_GroupInvite2),
 N(OP_GroupLeaderChange),
 N(OP_GroupLeadershipAAUpdate),
 N(OP_GroupMakeLeader),
+N(OP_GroupMentor),
 N(OP_GroupRoles),
 N(OP_GroupUpdate),
 N(OP_GroupUpdateB),