Grade posted status message missing "day"

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

Note: the output for this in the myactivities feed still doesn't look right.  The source value is 

     "source": "Bearfacts:EndOfTermGrades",

but that is what literally appears on screen.  There's pending missing translation to a human-readable label.