Allow admin users to create or modify users without a shared secret (#6495)

changelog.d/5742.feature

docs/admin_api/user_admin_api.rst

synapse/handlers/admin.py

synapse/rest/admin/__init__.py

synapse/rest/admin/users.py

synapse/storage/data_stores/main/registration.py

tests/rest/admin/test_admin.py

tests/rest/admin/test_user.py

tests/storage/test_registration.py