Tags meta box: add a `button` ARIA role to the tag cloud links

[afercia]

See related #27555 and #26504.

For better accessibility, the tag cloud links in the Tags meta box should be reported to assistive technologies as buttons.

They're not real links because activating them doesn't navigate to a new resource, instead it performs an action. The tag cloud is printed out in different contexts though so the ARIA role should be added just on the Tags meta box. I'd propose to check for the links href attribute, when it is set to #.

[afercia]

First patch. Setting milestone to 4.7 as this would be a nice addition to the improvements proposed in #27555.

[afercia]

In 38800:

Editor: Add a role button to the Tags meta box tag cloud links.

For better accessibility, the tag cloud links in the Tags meta box should be
reported to assistive technologies as buttons. They don't navigate to a new
resource, instead they perform an action.

Fixes #38318.