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.

synapse/rest/saml2/metadata_resource.py

@@ -0,0 +1,36 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+import saml2.metadata
+
+from twisted.web.resource import Resource
+
+
+class SAML2MetadataResource(Resource):
+    """A Twisted web resource which renders the SAML metadata"""
+
+    isLeaf = 1
+
+    def __init__(self, hs):
+        Resource.__init__(self)
+        self.sp_config = hs.config.saml2_sp_config
+
+    def render_GET(self, request):
+        metadata_xml = saml2.metadata.create_metadata_string(
+            configfile=None, config=self.sp_config,
+        )
+        request.setHeader(b"Content-Type", b"text/xml; charset=utf-8")
+        return metadata_xml