Use task names to reduce the risk of duplicate update-feed tasks.

The task name is based on the feed id (URL). Setting this should reduce the risk of duplicate update-feed tasks (#231) because the queue shouldn't accept tasks with duplicate names. According to the docs this isn't an iron guarantee, but should work under most circumstances.

I've been running this change self-hosted for a day or so. I'm not sure whether or not it's caught any duplicates, but it doesn't seem to cause any new problems.
Why not just `base64.URLEncoding.EncodeToString([]byte(id))`?