Enable external link checking

[elichad]

We do not currently validate that external links work, so occasionally broken links end up on the live site. htmlproofer (which is used in our CI tests) can check external links, it's just disabled currently.

Now that our internal links are in order, we should enable external link validation too.

[elichad]

Broken links can be found here, for example:

• https://www.researchobject.org/ro-crate/desci the detached RO-Crate link
• https://www.researchobject.org/ro-crate/wildlive the example crate links