Title-based preview images for social networks

[samdark]

It was introduced for habr.com is looks more attractive than using single image or no image.

• [ ] When an article is saved image should be generated and stored in a file system. We could save to public directory.
• [ ] Image meta-tags (https://github.com/yiisoft-contrib/yiiframework.com/blob/master/views/layouts/main.php#L73 and https://github.com/yiisoft-contrib/yiiframework.com/blob/master/views/layouts/main.php#L81) should be generated dynamically.
• [ ] We can consider using these images in RSS https://github.com/yiisoft-contrib/yiiframework.com/blob/master/controllers/RssController.php