Add a Cluster and Add an End of Cluster

A cluster represents a set of questions from which one or more may be randomly chosen. Clusters should be completed with an End of Cluster page for best results (otherwise they treat the End of Lesson as the EOC). Questions within a cluster are randomly selected by choosing "Unseen Question within a Cluster" as a jump. Questions within a cluster may either link to the EOC to exit the cluster, or jump to an unseen question within the cluster, or jump to any other page in the lesson. This also enables the creation of scenarios with a random element using the lesson module.