Implementing bulk updating for headings sequence rule.

[jcflow]

Enable Bulk Updating in A11y Checker

closes MAT-936

flag=none

test plan:

- Navigate to a RCE instance.
- Navigate to a RCE instance.
- Add the folowing example HTML.
<h2>Here</h2><h4>is</h4><h6>a</h6><p style="color:white">text</p>
- Notice that the a11y checker has three issues.
> Verify that when selecting "Change heading..." or "Remove heading..."
a checkbox for bulk update appears.
- Select checkbox.
- Submit button.
> Verify that the headings sequence issues was solved and
only the color constrast issue is missing.

For testing pull the branch, run "yarn build", copy lib/ folder inside canvas-lms/tinymce-a11y-checker/lib/ and run webpack (inside canvas) again. You can check that the files were updated using Chrome source viewer in committed files.

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[eschiebel]

Try this as your html

<p>this is <span style="color: #ced4d9;">another a11y </span>issue</p>
<h2>Here</h2>
<h4>is</h4>
<h6>a</h6>
<p><span style="color: #ced4d9;">text</span></p>

and then try to bulk update the headings, and it does not do them all. It seems like if there's a violation before the heading level issue, the bulk update does not complete