html security advisory pages

Provide html versions of the security advisories, which
just include the advisories as preformatted text.
Security advisories front matter

Intial pass at setting front matter on the security advisory pages
to generate the listings, tables, and html pages.
Add cve numbers to security advisory front matter
Security advisory patches link generation

Some advisories have multiple patches, so make a list of patches
in the sa front matter.

Configure a canonical base url for generating links for patches
and advisories.