upgrade tinymce-a11y-checker to v2.1.0
upgrade tinymce-a11y-checker to v2.1.0

fixes CORE-2505
fixes CORE-1747
fixes CORE-2847

test plan:
- create a new quiz
- enter an image without alt text in the rce
- run the checker, it should find the issue
- go to questions and add one
- enter an image without alt text in the rce
- run the checker, it should find the issue in the correct editor
- add an additional error to the editor
- run the checker, fix the error and click "Apply"
- focus should move to the close icon
- add an image by clicking on a thumbnail in the images sidebar
- run the checker, it should have an alt text filename error for the
  image

Change-Id: I3c81251c0fa11fa0637d59b01ef2b40e670eae01
Reviewed-on: https://gerrit.instructure.com/195733
Reviewed-by: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
Tested-by: Jenkins
Product-Review: Brent Burgoyne <bccf16d7abdd2611580b1cd7d7a6fcc150d619da@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms