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/cas.py

@@ -37,7 +37,7 @@ class CasConfig(Config):
 
             # The public baseurl is required because it is used by the redirect
             # template.
-            public_baseurl = self.public_baseurl
+            public_baseurl = self.root.server.public_baseurl
             if not public_baseurl:
                 raise ConfigError("cas_config requires a public_baseurl to be set")