Require direct references to configuration variables. (#10985)

This removes the magic allowing accessing configurable
variables directly from the config object. It is now required
that a specific configuration class is used (e.g. `config.foo`
must be replaced with `config.server.foo`).

synapse/config/saml2.py

@@ -199,7 +199,7 @@ class SAML2Config(Config):
         """
         import saml2
 
-        public_baseurl = self.public_baseurl
+        public_baseurl = self.root.server.public_baseurl
         if public_baseurl is None:
             raise ConfigError("saml2_config requires a public_baseurl to be set")