Add set and hash of feeds to Redis to improve subscriptions

[pstuifzand]

The subscriptions of the feeds, are kept in keys named "feed:<id>", but these are difficult to reference at the moment. There should be a set of "feed:<id>" items and a hash of urls to "feed:<id>". That way I can deduplicate feeds and make it easier to resubscribe.

(Originally published at: https://p83.nl/p/785)