Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
While sets are cool, I think this whole routine could be done in a simpler, and more efficient manner.  For example:

```
        existing_bug_ids = BugzillaBug.objects.filter(tasks__batch__query__exact=query).values_list('bugzilla_id', flat=True)

        def fetch(query, limit, offset):
            new_bugs = request_bugs(query.split('?')[1], limit=limit, offset=offset)
            return [bug for bug in new_bugs if bug['id'] not in existing_bug_ids]

```