Add aria attributes to improve facet accessibility.

[JakePT]

Description of the Change

Adds aria attributes to facets to improve accessibility. This includes adding aria-disabled and aria-role attributes to disabled links, as per the solution described in this article. It also includes adding an aria-label attribute to the links, so that the accessible label makes it clear whether following the link will add or remove the filter.

The change also includes a filter for the accessible label: ep_facet_widget_term_accessible_label.

Closes #2596

Verification Process

1. Add a facet widget to a page.
2. Observe the list of links, available facets should have an aria-label that reads "Apply filter:", followed by the term name.
3. After applying a filter, observe disabled options, they should have an aria-disabled="true" attribute and an aria-role="link" attribute. Active filters should have an aria-label that reads "Remove filter:", followed by the term name.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Added ARIA attributes to Facet widget links to improve accessibility.

Credits

Props @JakePT