Automatically delete feeds that have no subscribers and their cached articles after a waiting period has elapsed

[connorkuehl]

For example, the feeds table could grow a delete_after column which goose can set when it notices a feed has no subscriptions. Then, periodically, goose can check to see if there are any feeds whose delete_after time is in the past and then delete the feed's cached articles and the feed itself.

When adding a new subscription, the feed's delete_after column will have to be set back to NULL.

The delete_after interval could be set to something like 7 days.