Mark external links using the "rel" attribute

This will help search engines know these links point to external sites
and it'll make it possible to style these links using the
`[rel~=external]` selector. AFAIK, assistive techonologies don't use
this attribute to notify people about external links, though.

spec/components/layout/footer_component_spec.rb

@@ -6,7 +6,9 @@ describe Layout::FooterComponent do
       render_inline Layout::FooterComponent.new
 
       page.find(".info") do |info|
-        expect(info).to have_css "a[rel=nofollow]", count: 2
+        expect(info).to have_css "a", count: 2
+        expect(info).to have_css "a[rel~=nofollow]", count: 2
+        expect(info).to have_css "a[rel~=external]", count: 2
         expect(info).not_to have_css "a[target]"
       end
     end