Fix registration on workers (#4682)

* Move RegistrationHandler init to HomeServer

* Move post registration actions to RegistrationHandler

* Add post regisration replication endpoint

* Newsfile

synapse/module_api/__init__.py

@@ -79,7 +79,7 @@ class ModuleApi(object):
         Returns:
             Deferred: a 2-tuple of (user_id, access_token)
         """
-        reg = self.hs.get_handlers().registration_handler
+        reg = self.hs.get_registration_handler()
         return reg.register(localpart=localpart)
 
     @defer.inlineCallbacks