Require static images with js/require in cljs

• re-natal use-figwheel now scans for images and for each image adds require to index.*.js
• removed macro env.require-img/require-img

Upgrade existing projects

1. run re-natal upgrade
2. in cljs code replace all require-img calls to js/require
3. run re-natal use-figwheel
4. delete env/prod/env/require_img.clj