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)

Based on more conversation with Liz, I don't think it makes sense to predefine criteria. (Aside: we could predefine fields and values, or instead of predefining, we could provide autocomplete or validation in the create-batch form based on this: https://bugzilla.mozilla.org/config.cgi?ctype=json. _Notably, only a few of the fields are of interest to task admins, but some of them are temporary fields like `cf_status_firefox31` and Desktop QA are often tweaking their use of fields/flags so some flexibility may be needed._)