CYB-343 Engagement index: percentile is being calculated based on visible scores only
  * provides the student's percentile ranking in the engagement_index feed, as [:percentile] on
      the hash of students
  * implemented as a Strategy pattern, using refinements on Array

https://jira.ets.berkeley.edu/jira/browse/CYB-343
CYB-361 - Front-end follow up - percentile calculated on the back-end

https://jira.ets.berkeley.edu/jira/browse/CYB-361
Merge pull request #216 from christianv/CYB-361

CYB-361 - Front-end follow up - percentile calculated on the back-end