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)

Not sure if `is_draft` and `is_valid` should be included in `TaskTemplate`. If an admin starts the process of importing tasks but marks the task template as a draft, the associated tasks should not be created? If an admin marks a task template invalid, should all associated tasks become invalid?