The modification date helps track if an advisory was changed after
initial publication. This is especially important for OSV users who need
the modification date in the vulns.json index to determine whether an
existing advisory was updated so they can download the update.  Also,
keep "ref" (pointing to bug number) in all advisories, not just the TODO
ones.

An argument to output_pages() was missing. This command probably never
worked.

An advisory must come with at least one fixed package.

This generates templated files using some parallelism, reducing the
total mksite time to less than half in my tests. Increasing parallelism
even further is possible, but would make the code harder to understand.
The obvious technique of generating each templated file in its own
process is actually far slower because the overhead of process creation
dwarfs the time spent processing the template, which is on average very
small and quick.