Django-stubs

- Fix importing of `django.views` package
- `blank=True` doesn't make `CharField` nullable now, thanks to Naddiseo

- Add linting
- Fix annotations for `MultiValueDict`
- Remove dependency on Django itself

- Plugin support for `FormMixin`'s `get_form()` and `get_form_class()`
- Proper annotations for `QuerySet.in_bulk()`, thanks to syastrov

- `transaction.atomic` reworked, thanks to syastrov
- Add support for referring to models in the same file by name string in related fields

- Add `strict_optional = True` support

- Return proper types from `django.shortcuts.get_object_or_404` and `django.shortcuts.get_list_or_404`. https://github.com/mkurnikov/django-stubs/issues/22
- Add plugin support for `django.contrib.auth.get_user_model()` https://github.com/mkurnikov/django-stubs/issues/16
- Do not give "Unexpected attribute ..." false positive, if returned field type is `Any`.
- Add support for default related managers https://github.com/mkurnikov/django-stubs/issues/18.