model: Rename 'private' to 'direct' in notifications.

This commit replaces the use of 'private' with 'direct' in
notifications when the user sets the 'Include content of direct
messages in desktop notifications' setting to False.

Test updated.

Fixes part of #1288.

tests/model/test_model.py

@@ -2026,7 +2026,7 @@ def test_notify_users_enabled(
 
     @pytest.mark.parametrize(
         "hide_content, expected_content",
-        [(True, "New private message from Foo Foo"), (False, "private content here.")],
+        [(True, "New direct message from Foo Foo"), (False, "private content here.")],
     )
     def test_notify_users_hides_PM_content_based_on_user_setting(
         self, mocker, model, private_message_fixture, hide_content, expected_content

zulipterminal/model.py

@@ -1436,7 +1436,7 @@ def notify_user(self, message: Message) -> str:
                 ]
                 recipient = ", ".join(extra_targets)
             if not self.user_settings()["pm_content_in_desktop_notifications"]:
-                content = f"New private message from {message['sender_full_name']}"
+                content = f"New direct message from {message['sender_full_name']}"
                 hidden_content = True
         elif message["type"] == "stream":
             stream_id = message["stream_id"]