cubicweb #10011539 Add js script with defer and async attributes [open]

in the uiprops JAVASCRIPT definition mechanism it would be useful to have extra attributes so that one can add a JS file with "defer/async" attributes that tell the browser to defer the loading/execution of the file.

cf http://devdocs.io/html/element/script

prioritynormal
typeenhancement
done in<not specified>
load0.500
load left0.500
closed by<not specified>