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)

I don't really see this as a many-to-many. I think each `TaskInvalidationCriteria` should be assigned to a unique `TaskImportBatch`. It doesn't really make sense to try to share them.