email mirror: make being one of many recipients okay.

Previously the email gateway had to be the only address in a recipient
field or we'd mis-parse the recipient.

This commit also makes the mirror correctly handle addresses of the
form "Jessica McKellar <jesstess@zulip.com>".

(imported from commit 7435f2b59b8f47dc599cc869f64597a730af7d12)

zerver/management/commands/email-mirror.py

@@ -18,6 +18,7 @@ Run this management command out of a cron job.
 import email
 from os import path
 from email.header import decode_header
+from email.utils import getaddresses
 import logging
 import re
 import sys
@@ -159,9 +160,11 @@ def delete(result, proto):
 def find_emailgateway_recipient(message):
     # We can't use Delivered-To; that is emailgateway@zulip.com.
     for header in ("To", "Cc", "Bcc"):
-        recipient = message.get(header)
-        if recipient and recipient.lower().endswith("@streams.zulip.com"):
-            return recipient
+        recipients = getaddresses(message.get_all(header, []))
+        for recipient_name, recipient_email in recipients:
+            if recipient_email.lower().endswith("@streams.zulip.com"):
+                return recipient_email
+
     raise ZulipEmailForwardError("Missing recipient @streams.zulip.com")
 
 def fetch(result, proto, mailboxes):