add option to display students by sortable name in gradebook

when the feature flag is turned on, student names will be
displayed as their sortable name in the Gradebook. The default
sortable name is 'Last, First' - so 99% of the time this change will
flip the displayed names from 'First, Last' to 'Last, First' in the
Gradebook (unless the sortable name has been explicitly changed in
settings).

test plan:

1. As a teacher, view the Gradebook for a course that has at least 2-3 students in it.
2. Notice the student names are displayed as 'First-Name Last-Name'
3. Go into the course's Settings --> Feature Options, and turn the
    'Gradebook - List Students by Sortable Name' feature on.
4. Visit the Default Gradebook, and notice the student names are displayed
    as their sortable names (sortable name is  "Last-Name, First-Name" by default)
5. Visit the Individual View Gradebook, and click the dropdown to select a student
    in the 'Content Selection' section. Notice the student names are displayed by
    their sortable names ('Last-Name, First-Name' by default).

Note: In the Individual View Gradebook, in the 'Student Information' section, the
  student name will always appear as 'First-Name Last-Name'. I figured we would only
  want to show 'Last, First' when the name appears in a list. If we want to display
  the name as 'Last, First' in the 'Student Information' section when the feature
  is toggled on, I can certainly make that change.

closes CNVS-8947

Change-Id: Ia3c6ae39b53d3762a8847445c90c9c16195bf336
Reviewed-on: https://gerrit.instructure.com/44121
Product-Review: Hilary Scharton <hilary@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>