drag/drop files from dandiarchive assets browser INTO notebooks/widget

[yarikoptic]

The idea came while thinking about that iframe issue dandi/dandiarchive-legacy#745 : if there is a way to have in notebook (or its widget) to react on "drop" event -- it would be very handy to be able to have web-ui navigation pane in the left and then be able to just drag/drop elements into a widget or notebook which would react by e.g. updating or creating a new widget to browse.

ping @bendichter @waxlamp since might be of interest and might not be monitoring this repo.