Implement ARIA recommendations from Notebooks for All

[gabalafou]

The Notebooks for All project did research into ways to improve notebook HTML markup for disabled users.

@tonyfast put out a specification to make notebook markup more accessible.

Let's modify the @jupyterlab/notebook package so that it outputs HTML based on those recommendations.

This will affect the HTML markup in both:

• JupyterLab
• Notebook 7

@afshin and @gabalafou (me) have volunteered to work on a PR for this issue.

[tonyfast]

i've placed my recommendations in jupyter/notebook#6956 . it is quite a long tedious issue.

i do want to disclose that i've removed the feed pattern from the interactive version and we've assigned cells as region landmarks