update_message: Use `rendering_only` flag for notifications logic.
update_message: Use `rendering_only` flag for notifications logic.

When `update_message` events were updated to have a consistent
format for both normal message updates/edits and special
rendering preview updates, the logic used in the tornado event
queue processor to identify the special events for sending
notifications no longer applied.

Updates that logic to use the `rendering_only` flag (if present)
that was added to the `update_message` event format to identify
if the event processor should potentially send notifications to
users.

For upgrade compatibility, if `rendering_only` flag is not present,
uses previous event structure and checks for the absence of the
`user_id` property, which indicated the special rendering preview
updates.

Fixes #16022.

rht/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip