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 and add db_index

Implement delete_old_articles and separate cron job
Merge pull request #2381 from jgmize/openstandard-articles-multiple-categories

Store mult categories per article: fix bug 1083256