Support creation of aggregate Targets and Indicators based on Events

In MDL-62293, a UI allowing construction of simple learning analytic models based on Events is proposed. These indicators would all be binary. This proposal expands that interface by allowing the creation of "aggregate" Targets and Indicators based on events, e.g. a count of the number of times a user has posted content to an activity, or the average grade achieved by students in a course. The interface allows aggregations of events to be centered and scaled between -1 and 1 for inclusion in models.