Accessibility issues in revisions screen

[joedolson]

In the course of assessing a functional problem with wp_text_diff, a number of accessibility concerns were unearthed regarding the revisions screen.

Once the output of wp_text_diff is settled, the other concerns noted in #25473 should be addressed here.

[sarahricker]

Hey @joedolson Would you be able to list and summarize the final a11y to-do's from #25473 in the description? Thanks!

[joedolson]

Issues that still need to be resolved in the Revisions screen:

1) Can this be reduced to a single table, so that a single set of headers ('Removed' and 'Added') can be displayed for all content, rather than having different sets for each type of data.

2) Add aria-labelledby and aria-describedby attributes to the view slider to improve the accessibility of navigating revisions.

3) Fix headings hierarchy (current H3 headings should be H2)

[rishishah]

Here is the patch for "Fix headings hierarchy (current H3 headings should be H2)"