Implemented back-chanel logout

canaille/ldap_backend/schemas/oauth2-openldap.ldif

canaille/ldap_backend/schemas/oauth2-openldap.schema

canaille/oidc/clients.py

canaille/oidc/forms.py

canaille/oidc/models.py

canaille/oidc/oauth.py

canaille/templates/oidc/user/logout.html

demo/client/__init__.py

demo/ldif/bootstrap-data.ldif

tests/oidc/conftest.py

tests/oidc/test_client_admin.py

tests/oidc/test_end_session.py