FinalGradesTranslator was returning a nil source.  Added test that checks that the source is equal to Bearfacts:EndOfTermGrades and changed output of final_grades_translator to make it pass

The correct value for the source for both the final_grades _and_ the reg_status translator notifications is "Bear Facts", says Eli, so I changed this in the tests and the lib files.  Maybe there's a way to make this more DRY, since the hardcoded label is in two places, but at least it works as expected now.