Channel.last_notify is the datetime of the most recent
notification sent via the channel. Channel.last_error is
the error message (blank if the delivery was successful).

In the integrations list, "Last Notification" column,
use these fields instead of looking up the most recent
Notification object. This saves some db queries,
and also fixes a subtle issue: if prunenotifications
cleans up all notifications for a given channel, the
"Last Notification" column would display "Never", which
would not be correct – not any more.