Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
First off, I just ran through the import process on my local copy and it seems to work very smoothly; nice job on making it work! (The admin-facing forms are getting pretty complex though, but that's more questioning the feature specs for the site rather than anything code-wise).

As for the actual, code, I agree that this is really hard to follow, but I don't know if multiple views is the answer. Multiple views makes it annoying to re-use the form objects, which you'll need for validation on both the preview and creation. Some thoughts:

- Get rid of that session stuff. Is it a list of bugs, or metadata about the bugs? Either re-query post-preview for the bugs (this is submitted often so don't worry about hitting Bugzilla often) or add that data to a hidden form input.
- `_update_stage` has some validation going on, that should probably be in a custom form for this view instead of the view itself.
- Calling this stuff `stage` itself is confusing, as it initially made me think that there was like a 5-step process. In actuality, your logic looks like: 1) Submit the form without a confirmation. 2) See a preview and resubmit the same form with a confirmation. 3) Process the form and show the result. Find a way to make that clearer.
- Consider removing the definition of invalidation criteria from this view. If you remove the formset stuff, this becomes a `FormView` with some extra logic in `form_valid` depending on if the preview confirmation is present or not. Post-creation you can show a view that handles only the formset for bug invalidation criteria, after the tasks have been created.

The last point I think is the biggest; remove the formset stuff from this view and make it a separate step and you massively simplify the whole process. 

(In a perfect world we wouldn't be making One and Done so complex as to monitor Bugzilla bugs for changes to close out tasks, but that's not a helpful answer. :P)