Fixes issues related to Students changing Cohorts

This PR fixes problems related to Students switching Cohorts, which happened in Klass #22.

The underlying problem was using the SE --> SA --> Student --> Cohort --> LC path, which becomes invalid once a Student changes Cohort.  The correct path is SE --> AE --> A --> Cohort --> LC, since the LC is associated with the AE (via the A) and not the SA (via the Student).

A migration to fix inconsistent SEs has been added.  Columns to clarify the Cohort/Section of the Student vs the Assignment have been added to the report.xls spreadsheet.  The Klass and Assignment grade reports have also been updated to pull information from the correct path.  The Assignment show page has been updated, and permission-related issues have been fixed.  These changes resulted in some dead code which was removed from `cohort.rb`.