Allow for site-wide setting for email address(es) to notify for all issues and updates

While #729632: Allow for site-wide issues email address default value is also worth doing, I think the real solution to our problem at sec.d.o is a global setting that's a (list of) address(es) that should always be notified on all issues and followups. Other than the ~5 lines for the FAPI array for the setting UI, it should be a 3 line patch to mail.inc as we're actually sending out the emails...