Implements admin API to lock an user (MSC3939) (#15870)

changelog.d/15870.feature

docs/admin_api/user_admin_api.md

docs/usage/configuration/config_documentation.md

synapse/_scripts/synapse_port_db.py

synapse/api/auth/__init__.py

synapse/api/auth/internal.py

synapse/api/auth/msc3861_delegated.py

synapse/api/errors.py

synapse/config/user_directory.py

synapse/handlers/admin.py

synapse/handlers/user_directory.py

synapse/rest/admin/users.py

synapse/rest/client/logout.py

synapse/storage/databases/main/registration.py

synapse/storage/databases/main/user_directory.py

...se/storage/schema/main/delta/80/01_users_alter_locked.sql

tests/api/test_auth.py

tests/rest/admin/test_user.py

tests/storage/test_registration.py