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 think it would be more readable if I could use the Python 2.7 dict comprehension syntax, but I really was trying for readability with very descriptive variable names, white space usage, etc. I also avoided `filter(lambda article: category in article.category, articles)` in favor of what I thought was the more readable (and less 'clever') list comprehension.