fixes GRADE-2309
test plan:
1. Visit New Gradebook
2. Set focus on any assignment column header link
3. Verify the focus indicator is visible
Change-Id: I59ba7ed979b5f0aed06b8c3618d0cddd7ef8d43b
Reviewed-on: https://gerrit.instructure.com/204250
Tested-by: Jenkins
Reviewed-by: Adrian Packel <apackel@instructure.com>
Reviewed-by: Keith Garner <kgarner@instructure.com>
QA-Review: Jeremy Neander <jneander@instructure.com>
Product-Review: Jonathan Fenton <jfenton@instructure.com>