get_recipient_ids: Include user ID for self-messages using DM group.

When using direct message groups as the preferred method for 1:1 or
self-messages, the get_recipient_ids function should include the
user ID for self-messages. Without this, the UI cannot detect who
was the recipient of the message.

zerver/lib/display_recipient.py

@@ -249,8 +249,9 @@ def get_recipient_ids(
             to = [recipient.type_id]
         else:
             to = []
-            for r in get_display_recipient(recipient):
+            recipients = get_display_recipient(recipient)
+            for r in recipients:
                 assert not isinstance(r, str)  # It will only be a string for streams
-                if r["id"] != user_profile_id:
+                if r["id"] != user_profile_id or len(recipients) == 1:
                     to.append(r["id"])
     return to, recipient_type_str