Implement SAML2 authentication (#4267)

This implements both a SAML2 metadata endpoint (at
`/_matrix/saml2/metadata.xml`), and a SAML2 response receiver (at
`/_matrix/saml2/authn_response`). If the SAML2 response matches what's been
configured, we complete the SSO login flow by redirecting to the client url
(aka `RelayState` in SAML2 jargon) with a login token.

What we don't yet have is anything to build a SAML2 request and redirect the
user to the identity provider. That is left as an exercise for the reader.

tests/utils.py

@@ -139,6 +139,7 @@ def default_config(name):
     config.admin_contact = None
     config.rc_messages_per_second = 10000
     config.rc_message_burst_count = 10000
+    config.saml2_enabled = False
 
     config.use_frozen_dicts = False