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
Did a bit more research, and as best I can tell, while there is a slight difference in the generated SQL, they are equivalent from both a logic and performance perspective.