External assignments

This should resolve issue #124.

A new External Assignments page hierarchy has been added off the Class show page.  This allows Instructors to enter grades from assignments issued outside OST and allows Researchers to get a report of those grades.

External Assignments (EAs) are composed to External Assignment Exercises (EAEs).  EAEs can reference Concepts and Topics to aid research-related analysis.  Students in the Class have associated Student External Assignments (SEAs) and Student External Assignment Exercises (SEAEs) to hold their scores.

Editing is largely done in-place (if the User is allowed) by clicking on names, strings, pulldowns, etc.