Opaque Keys fix for grade_summary

@carsongee  @sarina Just another tiny Opaque Keys fix in the legacy dashboard.
@sarina It's accessible from this url in `lms/urls.py`: 
```
        url(r'^courses/{}/legacy_grade_summary$'.format(settings.COURSE_ID_PATTERN),
            'instructor.views.legacy.grade_summary', name='grade_summary_legacy'),
```
With a corresponding button on the legacy instructor dashboard:
```
<a href="${reverse('grade_summary_legacy', kwargs=dict(course_id=course.id.to_deprecated_string()))}" class="${'is-disabled' if disable_buttons else ''}">${_("Grade summary")}</a>
```

One of our users tried to call it and saw a 500. It actually just displays an anticlimactic "not yet implemented" message. :)