Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
I'm not sure what you mean by "frozen data model needs to be updated." I just tested regenerating the migration in a branch from master that has my changes merged into it (`git checkout master; git checkout -b migrationtest; git merge bugzillataskimport; git rm $0012_migration_that_has_to_be_replaced`), and after the new migration everything works as before. Do I need to do anything else to update the frozen data model?