run-dev: Set HTTP header to show we're proxing from port 9991.

Previously, while Django code that relied on EXTERNAL_HOST and other
settings would know the Zulip server is actually on port 9991, the
upcoming Django SAML code in python-social-auth would end up detecting
a port of 9992 (the one the Django server is actually listening on).
We fix this using X-Forwarded-Port.

tools/run-dev.py

@@ -319,6 +319,8 @@ class CombineHandler(BaseWebsocketHandler):
         # type: () -> None
         if 'X-REAL-IP' not in self.request.headers:
             self.request.headers['X-REAL-IP'] = self.request.remote_ip
+        if 'X-FORWARDED_PORT' not in self.request.headers:
+            self.request.headers['X-FORWARDED-PORT'] = str(proxy_port)
         if self.request.headers.get("Upgrade", "").lower() == 'websocket':
             return super().prepare()
         url = transform_url(