security: Inter-link between SA and EN pages for greater visibility

Both sets of announcements are important for administrators to be aware
of, so make it more obvious that ENs exist when looking at SAs, and make
it clear that SAs aren't included in the list of ENs.

Suggested by: brooks
Reviewed by: brooks, emaste, jhb, pauamma_gundo.com
Differential Revision: https://reviews.freebsd.org/D42916