Refactor contact mailer

- Send single email with new email in `to` and old email in `bcc` fields
- Remove `Que::Mailer` (#895)
- Add preview
- DRY templates
- Add tests
- Extract translations
- Remove useless specs

test/mailers/contact_mailer_test.rb

@@ -0,0 +1,24 @@
+require 'test_helper'
+
+class ContactMailerTest < ActiveSupport::TestCase
+  include ActionMailer::TestHelper
+
+  setup do
+    @contact = contacts(:john)
+    ActionMailer::Base.deliveries.clear
+  end
+
+  def test_delivers_email_changed_email
+    assert_equal 'john-001', @contact.code
+    assert_equal 'john@inbox.test', @contact.email
+
+    email = ContactMailer.email_changed(contact: @contact, old_email: 'john-old@inbox.test')
+              .deliver_now
+
+    assert_emails 1
+    assert_equal %w[john@inbox.test], email.to
+    assert_equal %w[john-old@inbox.test], email.bcc
+    assert_equal 'Teie domeenide kontakt epostiaadress on muutunud' \
+                 ' / Contact e-mail addresses of your domains have changed [john-001]', email.subject
+  end
+end