Improve samples_factory generated objects.

Improve samples_factory generated objects.

* Add LibraryType and MultiplexIndex factories.
* Use factoryboy's fuzzy tools to generate more interesting objects,
  especially to avoid problems with Unique=True fields.
* Use django_get_or_create to make it easier to deail with things
  like usernames
* Define field values for is_staff and is_superuser for the user model
* Unsurprisingly this required updating some test cases to
  be compatible with the changes.