Added skeleton models for GradingSurveyGroup and Records. SurveyRecordGroup has been renamed to GradingRecord. GradingSurveyGroup will hold which Surveys should be linked. While the acompanying GradingRecords (with their reference to a GradingSurveyGroup) will hold the Records for each StudentProject that can take these Surveys.

# Primary owners
arb
bslatkin
guido

# Backup
kgibbs