Migrate from import task that stores the latest assessments on each Student record to read-time queries

[kevinrobinson]

eg, migrate this: https://github.com/studentinsights/studentinsights/blob/master/app/models/student.rb#L200 to an optimized read query (or we might not even need to optimize this and could just query for the data in Student#update_recent_student_assessments when it's needed).