Added validation at the model level so that user can subscribe to a feed only once.

Subscribing to a feed a second time should be stopped during request processing (currently at the Feed.subscribe method), but it doesn't hurt also making sure of it at the model level.
Ensure at the model level that a user cannot be associated with a folder more than once.
Ensure at the model level that a feed cannot be associated with the same folder more than once.
Ensure at the model level that a feed cannot be associated with the same user more than once.
Ensure at the model level that a feed cannot be associated with the same folder more than once (from the feed side).
Ensure at the model level that the same entry cannot be associated with the same feed more than once.
The responsability of finding out if a user is already subscribed to the feed he's trying to subscribe has been moved to the Feed.subscribe method.

If the user tries to subscribe to a feed a second time:

- Feed.subscribe will raise an AlreadySubscribeError.
- Feedscontroller will capture the error and return an HTTP 304 error.
- The javascript in the page will show a "you're already subscribed" alert.