A user can now subscribe to a feed that wasn't in the database before, submitting the feed URL (not the webpage URL, yet).
WIP: Add the capability for the FeedClient to perform feed autodiscovery when passed the URL of a webpage, instead of a feed URL. Improved FeedClient error handling.

Now FeedClient should log all errors raised, but errors should never bubble up. It returns true if it can successfully fetch the feed (this includes being returned a 304-Not Modified response from the server), and a false if anything goes wrong.