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/previews/contact_mailer_preview.rb

@@ -0,0 +1,13 @@
+class ContactMailerPreview < ActionMailer::Preview
+  def email_changed
+    # Replace with `Contact.in_use` once https://github.com/internetee/registry/pull/1146 is merged
+    contact = Contact.where('EXISTS(SELECT 1 FROM domains WHERE domains.registrant_id = contacts.id)
+                             OR
+                             EXISTS(SELECT 1 FROM domain_contacts WHERE domain_contacts.contact_id =
+                             contacts.id)')
+
+    contact = contact.where.not(email: nil, country_code: nil, code: nil).first
+
+    ContactMailer.email_changed(contact: contact, old_email: 'old@inbox.test')
+  end
+end