Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
@bobsilverberg I have not changed the flow of the import process, but I did refactor the view a bit to pull out parts of the "stage" stuff into a separate form and (hopefully) make all that conditional code clearer. 
All the other review comments have been addressed as well, except for unit tests, which are coming right up.