BaseMailer: strip <style> content from TextBody fix

If  ```$htmlLayout``` or ```$view``` contain ```<style>``` tags with CSS,
```strip_tags()``` here: https://github.com/yiisoft/yii2/blob/master/framework/mail/BaseMailer.php#L189
won't clean those CSS rules.

Actual html: 
```html
<html>
    <head>
        <style>
            .text-secondary {color: #93959A;}
            .gray-text{color:#aaa}
            .green-text{color:#2CB200}
            .red-text{color:#E92C13}
            [...]
        </style>
    </head>
    <body>
        <div>Content Block</div>
    </body>
</html>
```

Will look this way in TEXT version of Email: 
```
.text-secondary {color: #93959A;}
.text-secondary {color: #93959A;}
.gray-text{color:#aaa}
.green-text{color:#2CB200}
.red-text{color:#E92C13}
[...]
Content Block
```

--------------


When TEXT version of Email differs drastically from visible content of HTML version,
it can be result that some penalty points will apply to such mails: 
http://wiki.apache.org/spamassassin/Rules/MPART_ALT_DIFF_COUNT


Since it's default behaviour for both yii2-app-advanced and yii2-app-basic (where stripped from tags html used for TextBody),
I believe people will stumble in this for sure, without even knowing that their emails can be penalized.


You can reproduce it by placing CSS <style> blocks in email view or layout
and test it here: http://www.mail-tester.com/ (SpamAssassin section).