Create modal showing bulk add progress

Cribbed from @fofr's prototype.

Still needs:
* some tidying up
* the ability to dismiss it
* not to be shown after it is dismissed/the page is refreshed

* Spinner images from
http://www.mytreedb.com/view_blog/a-33-high_quality_ajax_loader_images.h
tml
* Shows an updating count and progress bar
* Doesn’t gracefully handle the adding/updating situation, needs
different copy in that situation