Enforce compatibility when processing make_join requests

Reject make_join requests from servers which do not support the room version.

Also include the room version in the response.

synapse/federation/transport/server.py

@@ -431,9 +431,31 @@ class FederationMakeJoinServlet(BaseFederationServlet):
     PATH = "/make_join/(?P<context>[^/]*)/(?P<user_id>[^/]*)"
 
     @defer.inlineCallbacks
-    def on_GET(self, origin, content, query, context, user_id):
+    def on_GET(self, origin, _content, query, context, user_id):
+        """
+        Args:
+            origin (unicode): The authenticated server_name of the calling server
+
+            _content (None): (GETs don't have bodies)
+
+            query (dict[bytes, list[bytes]]): Query params from the request.
+
+            **kwargs (dict[unicode, unicode]): the dict mapping keys to path
+                components as specified in the path match regexp.
+
+        Returns:
+            Deferred[(int, object)|None]: either (response code, response object) to
+                 return a JSON response, or None if the request has already been handled.
+        """
+        versions = query.get(b'ver')
+        if versions is not None:
+            supported_versions = [v.decode("utf-8") for v in versions]
+        else:
+            supported_versions = ["1"]
+
         content = yield self.handler.on_make_join_request(
             origin, context, user_id,
+            supported_versions=supported_versions,
         )
         defer.returnValue((200, content))