unlink many children from a directory at once

[zooko]

I have a directory with many children, and would like to remove them all. Currently I have to remove them one at a time, which is inconvenient, especially through the WUI. To close this ticket, add a method to dirnode.py named something like delete_multiple_children along with tests in test_dirnode.py. (Probably just copy class allmydata.test.test_dirnode.Deleter to create your new class MultiDeleter.)

Then add controls to the WUI and the cli to unlink multiple children at once.