Luigi improvements after feedback received about luigi HOT 1 CLOSED

Thanks for reviewing and creating action items!
I just wanted to remind about the possibility of importing authentication callback handler. Currently I have to copy js code from callback.html. I'd rather import the function than copy the whole file. You could also check if it could be achieved in another way i.e by enabling default oauth handler in oauth config. In my PR it was possible to import it.

Maybe the Luigi UMD action item already covers that feature request, but I just wanted to clarify it ;)

from luigi.