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)

There are two distinct tables for `Task` and `TaskTemplate`, both based on `Abstract task`. Tasks have a couple of extra fields. This brings up a question I have about `TaskKeyword` -- see below.