Not store the children of root skill

[elenadimitrova]

There are potentially some gas cost savings to be had from not adding the skill.children of the root skill as the children of that will always be all skills. See todo

[elenadimitrova]

addGlobalSkill costs ~278,608 gas and here we can save 25,000 or about 9% of that. However the additional logic needed for handling the root skill as a special case potentially out-weights this gas saving. Since this is non-essential for the Colony Contribute launch, we're iceboxing it for now.