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.
I think I'm going to change this to throw an exception instead of returning null, that way there's no way an error won't fall through and cause a weird bug somewhere else.