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
Also I would just do the easier:

```python
    link = article_data.pop('link')
    article, created = Article.objects.get_or_create(link=link, defaults=article_data)
    if not created:
        for field, value in article_data.items():
            setattr(article, field, value)
        article.save()
```