Allow Matomo cookies to be disabled

- Add `matomo_disable_cookies` setting and use it
  in the configuration of the Matomo script

app/views/layouts/_matomo.html.haml

 <!-- Matomo -->
+- matomo_disable_cookies = extra_config.has_key?('matomo_disable_cookies') && extra_config.matomo_disable_cookies
 = javascript_tag do
   :plain
     var _paq = window._paq = window._paq || [];
     _paq.push(['trackPageView']);
     _paq.push(['enableLinkTracking']);
+    #{matomo_disable_cookies ? '_paq.push(["disableCookies"])' : ""};
     (function() {
       var u="//#{extra_config.matomo_url}/";
       _paq.push(['setTrackerUrl', u+'matomo.php']);

changelogs/unreleased/271263-piwik-support-the-disabling-of-cookies.yml

+---
+title: 'Matomo: Support the disabling of cookies'
+merge_request: 52831
+author: 'otheus@gmail.com'
+type: added

config/gitlab.yml.example

@@ -1242,6 +1242,7 @@ production: &base
     ## Matomo analytics.
     # matomo_url: '_your_matomo_url'
     # matomo_site_id: '_your_matomo_site_id'
+    # matomo_disable_cookies: false
 
   rack_attack:
     git_basic_auth:

spec/views/layouts/_head.html.haml_spec.rb

@@ -102,6 +102,44 @@ RSpec.describe 'layouts/_head' do
       expect(rendered).to match(/<script.*>.*var u="\/\/#{matomo_host}\/".*<\/script>/m)
       expect(rendered).to match(%r(<noscript>.*<img src="//#{matomo_host}/matomo.php.*</noscript>))
     end
+
+    context 'matomo_disable_cookies' do
+      context 'when true' do
+        before do
+          stub_config(extra: { matomo_url: matomo_host, matomo_site_id: 12345, matomo_disable_cookies: true })
+        end
+
+        it 'disables cookies' do
+          render
+
+          expect(rendered).to include('_paq.push(["disableCookies"])')
+        end
+      end
+
+      context 'when false' do
+        before do
+          stub_config(extra: { matomo_url: matomo_host, matomo_site_id: 12345, matomo_disable_cookies: false })
+        end
+
+        it 'does not disable cookies' do
+          render
+
+          expect(rendered).not_to include('_paq.push(["disableCookies"])')
+        end
+      end
+
+      context 'when absent' do
+        before do
+          stub_config(extra: { matomo_url: matomo_host, matomo_site_id: 12345 })
+        end
+
+        it 'does not disable cookies' do
+          render
+
+          expect(rendered).not_to include('_paq.push(["disableCookies"])')
+        end
+      end
+    end
   end
 
   def stub_helper_with_safe_string(method)