Refactor external link styling to use external_links_class

[welpo]

IMPORTANT: Please do not create a Pull Request adding a new feature without discussing it first.

The place to discuss new features is the forum: https://zola.discourse.group/
If you want to add a new feature, please open a thread there first in the feature requests section.

Since this is a small refactor, I did not discuss it first. Feel free to close this if it's not wanted!

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Code changes

External link styling added in #2522 was implemented before the PR that added external link classes (#2717).

This PR follows up on both by refactoring the config so as to remove the external_links_no_referrer = true line, as it was just added as a workaround to style external links.

The styling is now done through the external class, by setting external_links_class = "external".

The site looks identical, before and after this change.

• Are you doing the PR on the next branch?

Since it's tiny and only affects the docs/site, I'm targeting master. Let me know if you'd prefer this in next.

[Keats]

you'll need to bump the zola version in https://github.com/getzola/zola/blob/maser/.github/workflows/docs.yml as well

[welpo]

Done!

[Keats]

Thanks!