[ticket/11896] "Mark all notifications read" does not work

form_time is only set if is passed via the form. Since the mark notifications
read link does not use the form, it will default to 0 causing the mark
notifications logic to only mark notifications read if their time is smaller
or equal to 0. This patch will change ucp_notifications to correctly set
form_time for the confirm_box.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11896

PHPBB3-11896