gradebook: focus selected section when opening popup

fixes CNVS-10420

test plan:
  * open gradebook in a course with multiple sections
  * open the section selection dropdown
  * select a section
  * reopen the dropdown
  * verify that the section you selected is focused

Change-Id: I18d7a346085fa77e1dcda6b8d17cda2f030b1b42
Reviewed-on: https://gerrit.instructure.com/28580
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>