remove fetch object, move it into Feed.fetch_from!
slug library copied from Discourse
turn feed names into a slug
Revert "turn feed names into a slug"

This reverts commit ad615bc86f45cfb61fad30f59ce85a77ad926702.
storage objects take a feed slug as an id
items are sorted by date
Feed changes:

* {Feed,Storage}.all class methods
* Feeds only contain their most recent items
* Feeds save themselves, not their most recent item
Fetch command, no specs