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)

Yeah, it's anything involving users, due the migration that allows users to be deleted. When I created this migration, South asked for default values for all these user-related fields in case of a backward migration, but it seems like that should have been taken care of already in the previous migration.