Automatically delete old notifications from notification_internal_activity

[Richard Mansfield]

The notification_internal_activity table can get pretty big after a while because we rely on users to delete their own notifications.

Some of the more useless notifications could be removed periodically to make upgrades on that table a bit less painful. See http://mahara.org/interaction/forum/topic.php?id=2287

The worst offenders that would be safe to delete are forum post, view access, watchlist, and institution message notifications, perhaps just when they're marked as read, and are a few months old.

Most of the 'maharamessage' and ones could go too when they're old, but because that type is a bit of a dumping ground for random system notifications, we'd have to be a bit careful about it.

Others such as the admin notifications and user messages, should just hang around forever because they're not stored anywhere else.

[Eugene (eugenev)]

Richard - I have written a patch that should get us going on this one ;).
I have added a core cronjob function that will delete stale (read and older than 21days) 'viewaccess', 'institutionmessage', 'watchlist', 'feedback' and 'newpost' notifications.

See the patch and let me know what you think.

[Eugene (eugenev)]

Richard - Please see the latest attached patch regarding this issue ;)

[Richard Mansfield (richard-mansfield)]

Looks good to me.