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 was wrong about `filter(on_homepage__isnull=True)` being equivalent b/c of a flaw in translating the double negative, but `filter(on_homepage__isnull=False)` generates a `WHERE "openstandard_article"."on_homepage" IS NOT NULL`, and `exclude(on_homepage=None)` generates a `WHERE NOT ("openstandard_article"."on_homepage" IS NULL)`