Bug 1030972 - Add ability to auto-import tasks from Bugzilla

Here are some tentative changes to the models, for feedback. Please see inline comments for some highlights. Diagram attached.
![importschema](https://cloud.githubusercontent.com/assets/6024326/3616075/f98f30ca-0dce-11e4-9437-51864ec0da18.png)

(Result of more conversation with Liz.) It doesn't make sense to mark a `TaskTemplate` invalid when it's created, true. But after a batch of tasks is imported, if the owner realizes that there's something horribly wrong with the whole batch, they can mark the `TaskTemplate` as invalid, which can then trigger an update of all the tasks from the batch to become invalid, instead of having to mark each task as invalid one by one.