Engagement Index now recognizes and can update current student

https://jira.ets.berkeley.edu/jira/browse/CYB-89
That would work, code-wise, or, even better, look at the strong params pattern as manifest in Api::V1::ActivitiesController -- this is a way that if someone were to craft a specific request it limits what it can do, e.g., fields not listed are not passed into the object creation.