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
If the script is unable to retrieve the original image during the feed parsing and write it to local disk, the ArticleImage should not be created in the db and the Article.image FK will be null, which should result in no image on the article, but not a broken image. The next time the script runs, it will try again.