When an URL in Decidim leads to an external domain a warning modal should appear when clicking to a link with the URL. There is a redesigned modal decidim-core/app/views/decidim/links/_invalid_url_modal.html.erb and also a deprecated modal decidim-core/app/views/decidim/links/_invalid_url_modal.html.erb making use of a highlight_domain helper method which is no longer required

• Fix the application to show the redesigned modal
• Adapt the tests at decidim-core/spec/system/external_domain_warning_spec.rb to check the warning with the correct modal
• Remove the deprecated modal, helper and test for helper