Replace !placeholder with @placeholder in mail code

Problem/Motivation

lib/Drupal/Core/Mail/MailManagerInterface.php:   *         $message['subject'] = t('Notification from !site', $variables, $options);
lib/Drupal/Core/Mail/MailManagerInterface.php:   *         $message['body'][] = t("Dear !username\n\nThere is new content available on the site.", $variables, $options);

modules/contact/contact.module:      $message['subject'] .= t('[!form] !subject', $variables, $options);
modules/contact/contact.module:      $message['body'][] = t("!sender-name (!sender-url) sent a message using the contact form at !form-url.", $variables, $options);
modules/contact/contact.module:      $message['subject'] .= t('[!form] !subject', $variables, $options);
modules/contact/contact.module:      $message['subject'] .= t('[!site-name] !subject', $variables, $options);
modules/contact/contact.module:      $message['body'][] = t('Hello !recipient-name,', $variables, $options);
modules/contact/contact.module:      $message['body'][] = t("!sender-name (!sender-url) has sent you a message via your contact form at !site-name.", $variables, $options);
modules/contact/contact.module:      $message['body'][] = t("If you don't want to receive such emails, you can change your settings at !recipient-edit-url.", $variables, $options);
modules/contact/src/MailHandler.php:      $sender_cloned->name = $this->t('!name (not verified)', array('!name' => $message->getSenderName()));
modules/update/update.module:  $message['subject'] .= t('New release(s) available for !site_name', array('!site_name' => \Drupal::config('system.site')->get('name')), array('langcode' => $langcode));
modules/update/update.module:    $message['body'][] = t('Your site is currently configured to send these emails when any updates are available. To get notified only for security updates, !url.', array('!url' => $settings_url));
modules/update/update.module:    $message['body'][] = t('Your site is currently configured to send these emails only when security updates are available. To get notified for any available updates, !url.', array('!url' => $settings_url));

Proposed resolution

* htmlToText already converts to txt
* Email subjects needs to remove all html.

Beta phase evaluation

See #2506427: [meta] !placeholder causes strings to be escaped and makes the sanitization API harder to understand.

Remaining tasks

User interface changes

Ideally, fewer completely unexpected double-escaping bugs.

API changes

None.