accessibility for rubric creating and editing

fixes CNVS-13130
fixes CNVS-13110

test plan
- as a teacher, go to /courses/:id/rubrics
- using screenreader, click 'Add Rubric'
- a ~magical~ rubric table should appear
- complete the tests for Criterion and Ratings below

- as a teacher, go to an assignment with a rubric
- verify that you can edit the rubric
- verify that you can delete it from the assignment
(no need to do a complete test run as below, just make sure the icons in the table header are accessible and that nothing fell down and died)

Criterion:
- the 'Edit' icon for the criterion description should be accessible
- click 'Edit' for the criterion description
  * click 'OK'; it should focus on the 'Edit' pencil
  * click 'Cancel'; it should focus on the 'Edit' pencil
- the 'Delete' icon for the criterion should be accessible

Ratings:
- the 'Edit', 'Delete', and 'Add' icons should be accessible
- verify that they read the correct messages ('Edit rating', 'Delete rating', 'Add new rating category after current rating')
- click 'Edit' for a rating
  * click 'OK'; it should focus back on the 'Edit' pencil
  * click 'Cancel'; it should focus back on the 'Edit' pencil
- click 'Add' for a new rating column; verify it focuses on that table cell
- click 'Delete' for your newly added rating; verify that focuses on the chunk of the table with ratings in it

- verify that adding and editing rubrics works just fine for keyboard only users
- celebrate, for rubrics are now ACCESSIBLE!!!!! \o/

:tada:

Change-Id: Ifbd3043f217c09cc7cb24e411b0a572fdce6ef52
Reviewed-on: https://gerrit.instructure.com/40639
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>