Use AR associations to write User.scores_by_team.
Fix gread <-> team association.
Add box-and-whisker of grade distribution per assignment.

Closes #272.
Fix test div and include.
Added routes for charts
Create table.
Fill table with charts.
Filter out assignments with no grades.
Add columns to table, set chart widths.
Fix ordering.
Create 'submitted_average' on Assignment.

Closes #369.
Linked assignments
Only show assignments with released grades in table.
Merge pull request #310 from bellotastudios/per-assign

Grade distribution per assignment.