Code refactoring, some optimizations and some fixes

Changes:

• Split project settings into separate modules (base, test).
• Running tests with DJANGO_SETTINGS_MODULE=config.settings.test is up to 30% faster than using the default settings module.
• Added default uwsgi.ini configuration file for reference.
• Moved wsgi module into config package.
• Changed behavior of updated_after and created_after filters; they only accept ISO formatted date and time values.
• Added more test cases (APIResourceTimestampableTestCaseMixin), up to 400 tests.
• Enabled Argon2 as password hasher. Reference: https://docs.djangoproject.com/en/dev/topics/auth/passwords/#using-argon2-with-django.