The encrypted push notification payload previously included a `pm_users`
field for group direct messages.
This commit replaces the `pm_users` field with `recipient_user_ids`.
(new name to migrate from the old "private" keyword)
It's a sorted array of all user IDs participating in a 1:1 or group DM
conversation, including both `user_id` and `sender_id`.
* Previously, `pm_users` was included only for group DMs;
`recipient_user_ids` is present for both 1:1 and group DM conversations.
* The old `pm_users` field was a string containing a comma-separated
list of sorted user IDs. `recipient_user_ids` has a more structured
array format.
Signed-off-by: Prakhar Pratyush <prakhar@zulip.com>