Hooked up Engagement Index with API

https://jira.ets.berkeley.edu/jira/browse/CYB-82

PR crucial for today's meeting

Hooked up @richcreationsrc Engagement Index with DataCulture's API
DataCultures can query it's DB and bring relevant information to Engagement Index
@students and @activitySums should be students and activitySums (or better, activity_sums), better to use locals over instance variables on self (which is an instance of the Controller here).  They are not referenced elsewhere, so they don't need to exist outside the scope of the index method.