DRYed the Feed model a bit.

The code that checks if a feed already is in the database, before subscribing a user to it, has been moved to its own method.
A user can subscribe to a feed already in the database by submitting its url or fetch_url, with or without trailing slash.

To enable this, the Feed.subscribe method searches for variants of the submitted URL with and without trailing slashes; if any matching feed is found, the user is subscribe to the already-known feed instead of trying to fetch a new feed from the internet.