Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
That's a good idea, I'll try it.

The `stage` stuff is inspired by the implementation of Django FormPreview [1] and Form wizard [2]. I essentially adapted the FormPreview approach to a view that involves multiple forms and FormSets.

[1] https://docs.djangoproject.com/en/1.4/ref/contrib/formtools/form-preview/
[2] https://docs.djangoproject.com/en/1.4/ref/contrib/formtools/form-wizard/