Django-smoke-tests

- improve how allowed and disallowed status codes are handled, improve a default rule

- drop support for Django 1.x, leave support for 2.2 LTS, 3.2 LTS only
- drop support for Python 2.x
- fix [15] - incorrect URLs generated for django2 style path() with parameters
- fix [17] - support namespaces in SKIP_SMOKE_TESTS

- fix documentation

- add support for Django 2.1

- fix [5] - use `get_user_model()`

- add `--fixture` parameter