Implement a web UI to show the queue status

[msm-cert]

After implementing #437 it would be nice to show the current indexing queue status somewhere.

I think the easiest solution would be to put this information in the status subpage (/status). It will be only visible to admin that way, but I don't think this feature warrants a new separate subpage (but if you think differently it's OK too).

The UI should show a list of queues (i.e. all ursadb instances with at least one file pending) and a number of pending files. Maybe also some other data, like oldest and newest file in each queue.

The UI should also have a button to remove all files (clear the queue).