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)

This is how I'm trying to accommodate keywords in TaskTemplates, but it seems... error prone. I still think it might be better to just have a "is_template" field in the Task table instead of having a distinct table for templates.