remote_server_email: Add 'unsubscribe link' in the mail footer.

Previously, the emails sent to the remote servers had the
'unsubscribe link' only present in the 'List-Unsubscribe' header.

Not all email clients expose that header.

So, this commit adds the link in the footer too.

templates/zerver/emails/custom_email_base.pre.html

@@ -14,6 +14,7 @@
 {% block manage_preferences %}
     {% if remote_server_email %}
     <p>You are receiving this email to update you about important changes to Zulip's Terms of Service.</p>
+    <a href="{{ unsubscribe_link }}">Unsubscribe</a>
     {% elif unsubscribe_link %}
     <p><a href="{{ realm_url }}/#settings/notifications">{{ _("Manage email preferences") }}</a> | <a href="{{ unsubscribe_link }}">{{ _("Unsubscribe from marketing emails") }}</a></p>
     {% endif %}

zerver/tests/test_email_notifications.py

@@ -89,6 +89,7 @@ class TestCustomEmails(ZulipTestCase):
             "You are receiving this email to update you about important changes to Zulip",
             str(msg.alternatives[0][0]),
         )
+        self.assertIn("Unsubscribe", str(msg.alternatives[0][0]))
 
     def test_send_custom_email_headers(self) -> None:
         hamlet = self.example_user("hamlet")