adds 'copy-to-clipboard' button for preview_link

[markconroy]

Closes #722

Here's a PR for #722, while we are waiting on the patch sent to Drupal.org to be committed.

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Thanks to Big Blue Door for sponsoring my time to work on this.

[stephen-cox]

This looks to work well. Although I have a couple of suggestions:

• The patch file contains sections for all the commits, it would be easier to read if these were all squashed like they are in https://git.drupalcode.org/project/preview_link/-/merge_requests/29.diff
• Very minor - maybe the button should be added between the link and the expiry date rather than after the expiry date, as it applies to the link.

[stephen-cox]

New patch looks good