Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
I replaced the TaskTemplate table stuff with this field after a preliminary review from @Osmose. I didn't actually use it because later discussion seemed to point to not having any notion of template at all. I think this needs to be discussed again by the team because there was some back and forth about it while you were away and I'm not sure what the final decision is.

The last time I talked to Liz about templates, she concluded that if someone wants to "repeat" a batch, they should do so by just cloning a task. (The batch-creation form would be pre-populated with data from the chosen task.)

Aside: I have task cloning implemented, but I want to resolve this PR before I make another PR for the cloning.