mattdrayer/api-course-key-conversion:  CourseKey data migrations, plus g...

...radebook history fix

@martynjames @chrisndodge -- data migration scripts to convert existing 'wrong-style' course identifiers back to slash-separated identifiers, which are supported by the new opaque key logic.  Also included here is a modification to StudentGradebookHistory which limits writes to only those gradebook updates which alter either grade or proforma grade.