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
These tests look fine, but most of the utils remain untested it seems, especially new ones. I'd like to have at least a good test of the new deletion function.