don't show rubric icons when tabbing through the page

hide rubric icons when tabbing through speedgrader,
and when tabbing through the individual rubric page.

closes CNVS-16436

test plan:

test plan:
- Have a course with at least 1 student
- Create an assignment that has a rubric attached
  (does not matter if the rubric is being used for grading or not)
- Have a student submit to the assignment
- In Speedgrader, view the rubric and verify the "+", "X"
  and pencil icons are no longer present when tabbing through the table.
  Also verify the icons are not present when hovering over the table.
- View the individual page for the rubric (for example,
  /courses/1/rubrics/1)
- Verify the "+", "X" and pencil icons are no longer present
  when tabbing through the table, before clicking the 'Edit Rubric'
  button. Also verify the icons are not present when hovering over
  the table.
- Click the 'Edit Rubric' button. Verify the icons show up when
  tabbing through the table. Also verify the icons show up when
  hovering over the table.

Change-Id: Id6c14fea24b9ed4c96a4ec826970798366a876a8
Reviewed-on: https://gerrit.instructure.com/43420
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>