allow a Vignette url to change formats after the url has been generated

@Wikia/platform-team @kvas-damian 
https://wikia-inc.atlassian.net/browse/PLATFORM-647

`ImagesService:: overrideThumbnailFormat()` is called after the url for an image has been generated, so we need a way to change the output format from a string :cry: 
Visually this is still a wall of text. What about something like:

```php
if (empty($thumbUrl) || !self::validThumbExtension($newExtension)) {
   return $thumbUrl;
}

if (VignetteRequest::isVignetteUrl($thumbUrl)) {
   $thumbUrl = VignetteRequest::setThumbnailFormat($thumbUrl, $newExtension);
} elseif (!self::imageUrlHasExtension($thumbUrl, $newExtension) && self::isDataTagImage($thumbUrl)) {
   $thumbUrl .= $newExtension;
}

return $thumbUrl;
```

I think part of what makes this hard is that there is no abstraction or interface for generating legacy thumbnail URLs consistently.