Refactor the usage of subsection block grades

[arslanashraf7]

Related to #3

We are currently using CourseGradeFactory to iterate over the enrolled students and their grades for Canvas sync purposes, as seen in a PR #8 (comment).

There is a probability that we can refactor our process of acquiring/syncing grades from CourseGradeFactory to Persistent. This might help us reduce a chunk of our code that we are currently using.

What Needs to be done?

• Refactor [CourseGradeFactory]https://github.com/edx/edx-platform/blob/master/lms/djangoapps/grades/course_grade_factory.py#L22 usage with PersistentGrades
• While doing the above also send assignment grades to canvas instead of the default value(1) as mentioned feat: add canvas integration plugin #8 (comment)