Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
Hmm, I suppose just cloning a task would work, but it would mean having to manually copy the query and description from an existing batch. It also seems less intuitive. I think ideally a user could clone a previous batch which would allow them to run the exact same import again. Or maybe even have a way of just running the same import again without creating a new batch - just attach the new tasks to the same batch.

@rbillings what do you think?