rework mod/ping and desktop notification logic

rework mod/ping and desktop notification logic

`mod/ping.php` returns all notifications ordered by date
mixing system notifications, private messages, intros, registrations
remove 'notify_full' mode
remove `&arr;` from unseen notes, 'seen' attrib now is "1" or "0"
new note attribute 'timestamp'
desktop notification logic use timestamp to check which notification show to the user
notification html template use timestamp to show full date to user in tooltip
`relative_date()` don't append full date on dates older than 24 hours