New MailHandler service for sending HTML emails

Commerce now has a MailHandler service (commerce.mail_handler) that should be used for sending HTML emails.

Allows a render array (with an associated #theme) to be used as the message body.
Ensures the correct translation language of both strings and entities.

Note: Since Drupal core doesn't support HTML emails out of the box, Commerce assumes that Swiftmailer (or an appropriate alternative) is used.

Code example:

    $subject = $this->t('Order #@number confirmed', ['@number' => $order->getOrderNumber()]);
    $body = [
      '#theme' => 'commerce_order_receipt',
      '#order_entity' => $order,
    ];
    // All parameters are optional.
    $params = [
      // Used for hook_mail_alter(). Prepended with "commerce_".
      'id' => 'order_receipt',
      // Defaults to the current store email.
      'from' => $order->getStore()->getEmail(),
      'bcc' => $order_type->getReceiptBcc(),
      // Defaults to the default language.
      'langcode' => $order->getCustomer()->getPreferredLangcode(),
    ];
    $this->mailHandler->sendMail($order->getEmail(), $subject, $body, $params);

Developers are encouraged to create a service per email under the Mail namespace. For example Drupal\commerce_order\Mail\OrderReceiptMail.