AX improvement: Reopen cookie popup should be a button instead of link

Problem/Motivation

After an accessibility review (by AnySurfer), we got informed that the link to reopen the cookie popup (#eucc-open) should be a button instead of a link.

Proposed resolution

See attached patch. The change done makes sure the functionality still works on existing links, but will also work on button tags with the data-eucc-open="true" attribute.

Example of button that can be added in the ckeditor:

<button class="button" role="button" aria-controls="sliding-popup" data-eucc-open="true">Cookie voorkeuren bijwerken</button>