Simplify how ruby-advisory-db is imported/converted to pages

[postmodern]

Simplify how ruby-advisory-db is imported and converted into pages. Instead of converting the YAML files into Markdown, simply inline the YAML into the Jekyll "front matter" and add layout: advisory. This will allow the actual conversion to be done by _layouts/advisory.html.

[postmodern]

We now have a advisories task which updates _advisories, re-generates the advisory files in advisories/_posts/ and commits the changes.