manages focus for assignments/discussion page menus

fixes CNVS-12718

test plan:
 - in a course with 2+ assignment groups
 - once with 1 assignment each, once with 2+ each
 - with draft state on
 - on assignment index page
 - using keyboard navigation for everything
 - click 'move to' for an assignment, close the dialog
 - verify that focus is directed to the settings cog for that assignment
 - repeat for an assignment group
 - click 'delete' for an assignment, click cancel
 - verify that focus is directed to the settings cog
 - click 'delete' for an assignment, click ok
 - for groups with 1 assignment: verify that focus goes to parent assignment group
 - for groups with 2+ assignments: verify that focus goes to previous assignment in list
 - for first assignment in list: verify that focus goes to parent assignment group

 - with draft state on
 - on discussion index page
 - repeat 'move to' and 'delete' verifications from above

Change-Id: I0940b5eed70cecb628c25c597b8439598d096d24
Reviewed-on: https://gerrit.instructure.com/35120
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>