Adds rel nofollow to external links (#4918)

* ✨ Adds rel nofollow to external links

Fixes #4583

Signed-off-by: Franck Nijhof <frenck@geekchimp.com>

* 🎀 Adds support for appending nofollow to existing external links

plugins/no_follow.rb

@@ -0,0 +1,33 @@
+# Jekyll Auto Nofollow Plugin
+# Automatically adds rel='external nofollow' to outgoing links.
+
+require 'jekyll'
+require 'nokogiri'
+
+module Jekyll
+    module NoFollow
+        def nofollow(content)
+            dom = Nokogiri::HTML.fragment(content)
+
+            # Find all links
+            dom.css('a').each do |link|
+                rel = ['external', 'nofollow']
+
+                # All external links start with 'http', skip when this one does not
+                next unless link.get_attribute('href') =~ /\Ahttp/i
+
+                # Play nice with our own links
+                next if link.get_attribute('href') =~ /\Ahttps?:\/\/\w*.?home-assistant.io/i
+
+                # Play nice with links that already have a rel attribute set
+                rel.unshift(link.get_attribute('rel'))
+
+                # Add rel attribute to link
+                link.set_attribute('rel', rel.join(' ').strip)
+            end
+            dom.to_s
+        end
+    end
+end
+
+Liquid::Template.register_filter(Jekyll::NoFollow)