Improve accessibility of Views table sortable columns

Problem/Motivation

Currently views table columns are read by screen readers in a way that is not helpful to users. A table column named "Status" would be read as "Status sort descending" which is both hard to understand and doesn't state the actual status of the column, only the action of the link. Some help text is given in a title tag, but title has been widely panned for no longer being an accessibility best practice:

• I thought title text improved accessibility, I was wrong.
• Don't rely on title attribute for accessibility & SEO
• Text Links: Best Practices for Screen Readers

Proposed resolution

I recommend both changing the title tag to one that's read: aria-label, and supplementing the help text with more descriptive content based on these examples:

• Assets.CMS.gov: Data Tables
• Inclusive Components: Data Tables

The aria-label on columns could work like:

(unsorted) 'Status' column. Activate to sort descending.
(active, sorted) 'Status' column, sorted descending. Activate to sort ascending.

The column name is wrapped in single quotes to be set off as not part of a sentence by the screen reader. If the column is sorted, the sort is indicated. Then the link action is indicated using "activate" instead of "click" to be pertinent to all users, whether they're navigating by keyboard, mouse, or another device.

Remaining tasks

I've tested in VoiceOver, but the behavior should be tested in other screen readers. The code isn't the DRYest... but it seemed like this was easier to read then chunking it up and concatenating the pieces together.

User interface changes

No visual changes, but changing what seems to be an unusable title tag into something more accessible.

API changes

None.

Data model changes

None.

Release notes snippet