Smarter refreshing via machine learning

[KeithCu]

Currently the system fetches usually every hour, or every 6 hours (for sites that usually update just once per day.) It does this 24/7.

It should be possible to apply some machine learning per feed to have the system figure out when the site usually updates, and then only make requests around then. This could be done manually (by keeping track of a week's worth of updates), or by applying some machine learning algorithms. It would be great if it could keep learning over time.

This would also be better for the sites that update once per day, because it could try to catch them soon after they are usually posted, rather than up to 6 hours later.