Add course, section, and professor-specific grade endpoints

[jpahm]

Currently, we have one centralized grade endpoint that handles all types of grade queries based on a provided subject, course number, and/or professor.

For example, a current grade request might look something like this:
GET /grades?subject_prefix=CS&course_number=1337

For the sake of backwards-compatibility and supporting mixed queries, it would be best to leave the current grades endpoint in place. That being said, it could be considerably more convenient for the following endpoints to also be created:

• Course-specific grades: GET /course/{courseId}/grades
• Section-specific grades: GET /section/{sectionId}/grades
• Professor-specific grades: GET /professor/{professorId}/grades

Ideally these endpoints will be able to share as much internal logic with the current /grades endpoint as possible so as to keep things uncluttered.

[dhomiller]

I'll take this one!

[jpahm]

I'll take this one!

It's all yours!