A bit under the weather, but got into building buttons to control running
and queued transfers today. The html and javascript side is done, with
each transfer now having a cancel button, as well as a pause/start button.

Canceling queued transfers works. Canceling running transfers will
need some more work, because killing a thread doesn't kill the processes
being run by that thread. So I'll have to make the assistant run separate
git-annex processes for transfers, that can be individually sent signals.