webadmin: add 30 px the the height of event notifier dialog.

Added height is needed so that the dialog warning will not overflow.
("Note: To receive email notifications, ensure that the mail server
is configured and the ovirt-event-notifier service is running.")

Change-Id: Ie249bc96f467ee765e579a134763d61e1aa4b29a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1122017
Signed-off-by: Mooli Tayer <mtayer@redhat.com>