Treat subdomains of the site host as internal links

[julioalucero]

Link to Jira

FR-767: Sibling subdomains get rel="nofollow" as if they were external links

PR description

SEMrush flagged 12 internal links across 9 pages of the FastRuby.io blog as nofollow. The nofollow isn't in the Jekyll sources, this gem adds it at build time: external_link? compared hosts exactly, so with url: https://www.fastruby.io the site host is fastruby.io and audit.fastruby.io / skunk.fastruby.io came out external. SEMrush counts subdomains as the same site. v0.2.0 already fixed the www. case, subdomains were the remaining gap.

• Added a subdomain check to external_link? in lib/jekyll-external-link-accessibility.rb: a host ending in .<site_host> is internal
  • The leading dot is what keeps notfastruby.io external. fastruby.github.io stays external too, which matters because a post links to it
  • Guarded on site_host being present, otherwise a site with no url would compare against "."
• Bumped VERSION to 0.3.0 (plus Gemfile.lock)
• Documented the new behavior in the README usage section

After this is released, the blog repo needs the gem bumped in Gemfile / Gemfile.lock / Gemfile.next.lock, and our_hosts in blog/spec/blog_spec.rb widened to any *.fastruby.io host so the same thing can't slip through again.

QA steps

1. Check the real hosts:

3Build the blog against this branch and confirm no nofollow on *.fastruby.io links, e.g. /blog/open-source-tooling.html had 3 of them

[etagwerker]

@julioalucero Good idea. Thanks!