Store mult categories per article: fix bug 1083256

Add Article.on_homepage field and associated cron task to avoid
race condition that could cause broken images if cache miss during
update_openstandard_feed cron job before images rsynced

Change Article.link to URLField

Implement delete_old_articles and separate cron job
I definitely agree that `filter` would be worse. I was more suggesting that it's not wrong to do things in more than one statement. No need to change it, but it was just giving off a hint of "fewer lines are better".