use DbGenericImportApp as a basis for DbAssignmentImportApp
use DbGenericImportApp as a basis for DbAssignmentImportApp

primary_key is not used in its db_update_method, so we overwrite it with
the column that is ("name")

jupyter/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,jupyter/nbgrader,jupyter/nbgrader