Add setting to require consent for notifications

Ensure GDPR compliance by default (Article 25 GDPR – privacy by design
and by default). Under GDPR, consent must be freely given, specific,
informed and unambiguous [1]. We were subscribing users without
explicity consent, which goes against the "No pre-ticked boxes"
principle.

For compatibility with existing installations, we're using a setting,
disabled by default. Once we release version 2.4.0 we will enable it by
default, which won't affect existing installations but only new ones.

[1] https://gdprinfo.eu/best-gdpr-newsletter-consent-examples-a-complete-guide-to-compliant-email-marketing

app/components/admin/settings/features_tab_component.rb

@@ -26,6 +26,7 @@ class Admin::Settings::FeaturesTabComponent < ApplicationComponent
       feature.sdg
       feature.machine_learning
       feature.remove_investments_supports
+      feature.gdpr.require_consent_for_notifications
       feature.dashboard.notification_emails
     ]
   end