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.

utils/sql/git/required/2014_10_18_group_mentor.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `group_leaders` ADD `mentoree` VARCHAR(64) NOT NULL;
+ALTER TABLE `group_leaders` ADD `mentor_percent` INT(4) DEFAULT 0 NOT NULL;