custom_email: Add manage_preferences block to the plaintext version.

Earlier, the content of the "manage_preferences" block that includes
the unsubscribe_link, personal settings link, etc was missing in the
plaintext version of the custom emails.

This commit updates the logic to include the manage_preferences block
content in the plaintext version.

templates/zerver/emails/custom_email_base.pre.manage_preferences_block.txt

@@ -0,0 +1,15 @@
+
+---
+{% if remote_server_email %}
+You are receiving this email to update you about important changes to Zulip's Terms of Service.
+
+Unsubscribe: {{ unsubscribe_link }}
+{% elif unsubscribe_link %}
+{{ _("Manage email preferences") }}:
+
+{{ realm_url }}/#settings/notifications
+
+{{ _("Unsubscribe from marketing emails") }}:
+
+{{ unsubscribe_link }}
+{% endif %}

zerver/lib/send_email.py

@@ -560,6 +560,16 @@ def custom_email_sender(
         #     vary user-to-user
         f.write(base_template.read().replace("{{ rendered_input }}", rendered_input))
 
+    # Add the manage_preferences block content in the plain_text template.
+    manage_preferences_block_template_path = (
+        "templates/zerver/emails/custom_email_base.pre.manage_preferences_block.txt"
+    )
+    with (
+        open(plain_text_template_path, "a") as f,
+        open(manage_preferences_block_template_path) as manage_preferences_block,
+    ):
+        f.write(manage_preferences_block.read())
+
     with open(subject_path, "w") as f:
         f.write(get_header(subject, parsed_email_template.get("subject"), "subject"))
 

zerver/tests/test_email_notifications.py

@@ -90,6 +90,11 @@ class TestCustomEmails(ZulipTestCase):
             str(msg.alternatives[0][0]),
         )
         self.assertIn("Unsubscribe", str(msg.alternatives[0][0]))
+        # Verify that the Text version contains the footer.
+        self.assertIn(
+            "You are receiving this email to update you about important changes to Zulip", msg.body
+        )
+        self.assertIn("Unsubscribe", msg.body)
 
     def test_send_custom_email_headers(self) -> None:
         hamlet = self.example_user("hamlet")
@@ -107,7 +112,7 @@ class TestCustomEmails(ZulipTestCase):
         msg = mail.outbox[0]
         self.assertEqual(msg.subject, "Test subject")
         self.assertFalse(msg.reply_to)
-        self.assertEqual("Test body", msg.body)
+        self.assertIn("Test body", msg.body)
 
     def test_send_custom_email_context(self) -> None:
         hamlet = self.example_user("hamlet")