Add confirmation step for email changes (#6071)

* Add confirmation step for email changes

This adds a confirmation step for email changes of existing users.
Like the initial account confirmation, a confirmation link is sent
to the new address.

Additionally, a notification is sent to the existing address when
the change is initiated. This message includes instruction to reset
the password immediately or to contact the instance admin if the
change was not initiated by the account owner.

Fixes #3871

* Add review fixes

spec/mailers/user_mailer_spec.rb

@@ -33,6 +33,20 @@ describe UserMailer, type: :mailer do
                      instance: Rails.configuration.x.local_domain
   end
 
+  describe 'reconfirmation_instructions' do
+    let(:mail) { UserMailer.confirmation_instructions(receiver, 'spec') }
+
+    it 'renders reconfirmation instructions' do
+      receiver.update!(email: 'new-email@example.com', locale: nil)
+      expect(mail.body.encoded).to include 'new-email@example.com'
+      expect(mail.body.encoded).to include 'spec'
+      expect(mail.body.encoded).to include Rails.configuration.x.local_domain
+      expect(mail.subject).to eq I18n.t('devise.mailer.reconfirmation_instructions.subject',
+                                        instance: Rails.configuration.x.local_domain,
+                                        locale: I18n.default_locale)
+    end
+  end
+
   describe 'reset_password_instructions' do
     let(:mail) { UserMailer.reset_password_instructions(receiver, 'spec') }
 
@@ -57,4 +71,16 @@ describe UserMailer, type: :mailer do
     include_examples 'localized subject',
                      'devise.mailer.password_change.subject'
   end
+
+  describe 'email_changed' do
+    let(:mail) { UserMailer.email_changed(receiver) }
+
+    it 'renders email change notification' do
+      receiver.update!(locale: nil)
+      expect(mail.body.encoded).to include receiver.email
+    end
+
+    include_examples 'localized subject',
+                     'devise.mailer.email_changed.subject'
+  end
 end