Remove user mapping and the default superuser group, plus related changes

README.md

ssoauth/__init__.py

ssoauth/app_settings/defaults.py

ssoauth/apps.py

ssoauth/auth_utils.py

ssoauth/checks.py

ssoauth/management/commands/create_compat_groups.py

ssoauth/management/commands/create_custom_groups.py

ssoauth/management/commands/group_mapping.py

...uth/management/commands/ssoauth_setup_groups_and_perms.py

ssoauth/migrations/0002_auto_20190306_1823.py

ssoauth/models.py

ssoauth/views.py