[FIX] portal_sale: re-add auto-subscribe of customer when confirming invoice

It was removed by mistake in commit [1]
when fixing the forward-port of 7.0
to saas-1. Only the mail notification
had to be removed, not the auto-follow
of the customer, which is important
in the portal, otherwise customers
cannot see their invoices.

[1] 8699 revid:chm@openerp.com-20130506135653-7e8nukr68s9255oq