Refactor JSNI code and move it outside of html5dnd.js when appropriate

[cindyloo]

The drag and drop code contains the JSNI interaction between java and js, in this case specifically project handlers like importProject. I think this is supposed to be an intermediary step as we move away from the GWT way of doing things and towards plain js. We should refactor this and other calls from the drag and drop code when appropriate

spawned from #3328

[ewpatton]

Note that this intersects with #3327.