Added max score to output of get_student_grade_summary_data.

This will put the actual score and actual max score in scope for
the the return_csv function, so actual scores can be dumped.

The ultimate goal is to provide this data in the CSV dump that is
passed to Stellar via pylmod.

This is PR #10395 on edX, and issue 95 on mitocw's edx fork.

https://github.com/edx/edx-platform/pull/10395

https://github.com/mitocw/edx-platform/issues/95