calculate density for svg thumbs

@drsnyder 
https://wikia-inc.atlassian.net/browse/PLATFORM-608

Calculate density so we can scale svgs up without causing blurry images. See current version of [vignette](http://vignette.wikia.nocookie.net/bloonsconception/images/2/25/Zoomtest.svg/revision/latest/thumbnail/width/200/height/200?replace=true) generated thumb vs [legacy](http://images.wikia.com/bloonsconception/images/thumb/2/25/Zoomtest.svg/200px-Zoomtest.svg) generated thumb.
nah, imagemagick can work with floats. The ones i saw were to ~10 decimal places.