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

spec/models/user_spec.rb

@@ -105,20 +105,44 @@ describe User do
     end
 
     describe "subscription_to_website_newsletter" do
-      it "is true by default" do
-        expect(subject.newsletter).to be true
+      it "is true by default when the consent for notifications setting is disabled" do
+        Setting["feature.gdpr.require_consent_for_notifications"] = false
+
+        expect(build(:user).newsletter).to be true
+      end
+
+      it "is false by default when the consent for notifications setting is enabled" do
+        Setting["feature.gdpr.require_consent_for_notifications"] = true
+
+        expect(build(:user).newsletter).to be false
       end
     end
 
     describe "email_digest" do
-      it "is true by default" do
-        expect(subject.email_digest).to be true
+      it "is true by default when the consent for notifications setting is disabled" do
+        Setting["feature.gdpr.require_consent_for_notifications"] = false
+
+        expect(build(:user).email_digest).to be true
+      end
+
+      it "is false by default when the consent for notifications setting is enabled" do
+        Setting["feature.gdpr.require_consent_for_notifications"] = true
+
+        expect(build(:user).email_digest).to be false
       end
     end
 
     describe "email_on_direct_message" do
-      it "is true by default" do
-        expect(subject.email_on_direct_message).to be true
+      it "is true by default when the consent for notifications setting is disabled" do
+        Setting["feature.gdpr.require_consent_for_notifications"] = false
+
+        expect(build(:user).email_on_direct_message).to be true
+      end
+
+      it "is false by default when the consent for notifications setting is enabled" do
+        Setting["feature.gdpr.require_consent_for_notifications"] = true
+
+        expect(build(:user).email_on_direct_message).to be false
       end
     end