set pprint/*print-right-margin* to popup window width

I am also changing the clojure form to return nil. Why would it return the object if it's used for side effects only? 
This has become a bit too long. Split the form string into several lines, so it's easier to read/comprehend. Examples to consider - https://github.com/clojure-emacs/cider/blob/master/cider-interaction.el#L1510 and https://github.com/clojure-emacs/cider/blob/master/cider-interaction.el#L1447 (which should be extracted into middleware, but still). Ideally we'll move this pretty-printing logic into the middleware layer as well.