adjust font size in gradebook2 settings menu

fixes CNVS-5525

test plan
- check items in settings wheel for gradebook2
- confirm the font sizes are the same
- confirm that text still fits
- double check that the 'Showing all Sections' menu looks correct

Change-Id: Id6a7ebd400bdf2aabfed7aa58b0989c4040d1b5e
Reviewed-on: https://gerrit.instructure.com/28971
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>