Pull students' challenge and exercise attempt data for Cohort show view

This data will be needed to calculate a last active date for each student.
Cohort show: days since each student's last submission displayed

On the Cohort show page, the number of days since the last challenge or exercise submission is displayed for each student.  This provides teachers a quick reference point for which students have and have not been recently active.
User show: days since student's last submission displayed

On the User show page, the number of days since the last challenge or exercise submission is displayed.  This provides teachers with a quick reference point as to whether or not the student has been recently active.
Merge pull request #23 from toreyhickman/last_active

Visibility into how long it's been since a student has submitted a challenge or exercise