Allow server admins to define and enforce a password policy (MSC2000). (#7118)

changelog.d/7118.feature

docs/sample_config.yaml

synapse/api/errors.py

synapse/config/password.py

synapse/handlers/password_policy.py

synapse/handlers/set_password.py

synapse/rest/__init__.py

synapse/rest/client/v2_alpha/password_policy.py

synapse/rest/client/v2_alpha/register.py

synapse/server.py

tests/rest/client/v2_alpha/test_password_policy.py