Cannot import for-purchase apps when archive_id=None

[Michael Nelson]

Currently the import_for_purchase_apps assumes that all published apps have a non-null archive_id, but this is no longer the case (as ARB apps can be published with a null archive_id). This leads to:

{{{
michael@server-7804:~/ca/webcat/current_work$ fab manage:'import_for_purchase_apps'
[localhost] local: virtualenv/bin/python django_project/manage.py import_for_purchase_apps
Traceback (most recent call last):
  File "django_project/manage.py", line 18, in <module>
    execute_manager(settings)
  File "/home/michael/ca/webcat/current_work/virtualenv/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 438, in execute_manager
    utility.execute()
  File "/home/michael/ca/webcat/current_work/virtualenv/local/lib/python2.7/site-packages/django_configglue-0.6.1-py2.7.egg/django_configglue/management/__init__.py", line 71, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/michael/ca/webcat/current_work/virtualenv/local/lib/python2.7/site-packages/django/core/management/base.py", line 191, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/michael/ca/webcat/current_work/virtualenv/local/lib/python2.7/site-packages/django/core/management/base.py", line 220, in execute
    output = self.handle(*args, **options)
  File "/home/michael/ca/webcat/current_work/src/webcatalog/management/commands/import_for_purchase_apps.py", line 69, in handle
    self.import_app_from_data(app_data, icon_data, distroseries)
  File "/home/michael/ca/webcat/current_work/src/webcatalog/management/commands/import_for_purchase_apps.py", line 73, in import_app_from_data
    form = ForPurchaseApplicationForm.from_json(app_data, distroseries)
  File "/home/michael/ca/webcat/current_work/src/webcatalog/forms.py", line 151, in from_json
    distroseries=distroseries)
  File "/home/michael/ca/webcat/current_work/virtualenv/local/lib/python2.7/site-packages/django/db/models/manager.py", line 132, in get
    return self.get_query_set().get(*args, **kwargs)
  File "/home/michael/ca/webcat/current_work/virtualenv/local/lib/python2.7/site-packages/django/db/models/query.py", line 351, in get
    % (self.model._meta.object_name, num, kwargs))
webcatalog.models.applications.MultipleObjectsReturned: get() returned more than one Application -- it returned 31792! Lookup parameters were {'archive_id': None, 'distroseries': <DistroSeries: Ubuntu Precise ()>}

Fatal error: local() encountered an error (return code 1) while executing 'virtualenv/bin/python django_project/manage.py import_for_purchase_apps '
}}}