Was updating it every time an item was updated, this was unnecessarily
slow. Typically, only a very small number of items are updated per run,
so it shouldn't make much of a difference, but a lot faster if many
items are updated.
Previously was using the quickbook file's pub_date, By using the date
the release script was run on, can release without updating the
quickbook file. So the release manger won't need to have quickbook
installed. At least in theory!
- Both are now stored as strings.
- Unpublished articles have a null pub_date.
- last_modified is now always the last modified date,
before it was the pub_date once published.
