create UrlGenerator from Vignette URL string

@drsnyder 
https://wikia-inc.atlassian.net/browse/PLATFORM-648

Second time that we need the ability to get/modify a thumbnail request after the url string has been generated. This makes it easier, since now we can create a `UrlGenerator` object from the string URL, and do any manipulation/fetching on the object. This PR also udpates `VignetteReqest::setThumbnailFormat` to use this fanciness.
Should this be a valid (but null-like) `UrlGenerator`? There is a call [above](https://github.com/Wikia/app/pull/5782/files#diff-b98dcbe1aa6bf8a7e0a32f5656c12158R175) that will throw an error in this case.