Debug module mail panel

For https://github.com/yiisoft/yii2/issues/1667
This PR allows you to view email messages that are saved if 
```php
public $fileTransport => true
```
You can view all the message parameters except body and attachments.
You can download the *. EML file messages.

During operation, I had a small question, I decided to ask it here and do not include to PR.
https://github.com/yiisoft/yii2/blob/6ed8d1767cf9d00dc4efae3ee3c52e8c386a9d87/framework/yii/mail/MessageInterface.php#L52
https://github.com/yiisoft/yii2/blob/6ed8d1767cf9d00dc4efae3ee3c52e8c386a9d87/framework/yii/mail/MessageInterface.php#L84

As indicated in phpdoc these two methods should return a string, but in the implementation
https://github.com/yiisoft/yii2/blob/41685a2409964add5c17b72e90e62615000d1665/extensions/yii/swiftmailer/Message.php
methods return arrays.

Sincerely!