Moved drupal_mail to MailManager::mail

drupal_mail() has moved to the MailManagerInterface::mail() method.

// D7:
drupal_mail($module, $key, $to, $langcode)

// D8:
\Drupal::service('plugin.manager.mail')->mail($module, $key, $to, $langcode);

In addition, some of the mail formatting helper functions were also moved to a helper class.

The drupal_html_to_text() function was moved to a static helper function MailFormatHelper::htmlToText()

// D7:
$text = drupal_html_to_text($string);

// D8:
$text = \Drupal\Core\Mail\MailFormatHelper::htmlToText($string);

drupal_wrap_mail() was moved to a static helper function MailFormatHelper::wrapMail().

// D7:
$text = drupal_wrap_mail($text);

// D8:
$text = Drupal\Core\Mail\MailFormatHelper::wrapMail($text)