ThemeDesigner custom background images

@drsnyder @owend 
https://wikia-inc.atlassian.net/browse/PLATFORM-564
`AssetsManager` takes in a query parameter of settings, some of which are urls. This creates a problem when a Vignette url is one of those settings (background image), since that url also has a query parameter. This sidesteps the issue by manually concatenating the parameters for `AssetsManagerSassBuilder`.
@macbre, @owend could we also use a reference to a File object here? Some concern has been expressed about not hitting the database in this code. Would that be an issue?