show correct student totals in speedgrader for separate sections

when viewing the speedgrader for a specific, filtered section
of a course, the total student count will now reflect the total
number of students in that section only. Previously, it was
reflecting the total student count in the entire course.

closes CNVS-16340

test plan:

- Have a course with 2 sections
- Add multiple students to each section
- Create an assignment in the course
- Sort the Gradebook by 1 of the sections
- Go to speedgrader of the assignment from the Gradebook
- Verify that speedgrader states you are looking at "student X of Y", where Y is the number of students in the section that was filtered for, NOT the number of students in the entire course (previous behavior was Y was equal to the number of students in the entire course).
- Also, create a course without multiple sections, and confirm the total student count is correct in speedgrader.

Change-Id: Ia701d5eb44e07c86718d4d0a889f5c14a4b0ac8e
Reviewed-on: https://gerrit.instructure.com/43575
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>