[OCTANE] Craft Tracked Properties RFC

Tracked properties introduce a simpler and more ergonomic system for tracking state change in Ember applications. By taking advantage of new JavaScript features, tracked properties allow Ember to reduce its API surface area while producing code that is both more intuitive and less error-prone.

This issue is currently in-progress (@pzuraq @tomdale) and is part of the Octane Edition of Ember.js. 