Hardcoded favicon path in favicon_link_tag helper call, in layouts.

This way there is no ambiguity; firefox seems to always request /favicon.ico, but the html header had /images/favicon.ico as the favicon path. Hardcoded the /favicon.ico path in the html header to ensure the favicon works and always uses the same path.
Global GUID uniqueness for entries is no longer enforced. GUIDs are in the scope of a single feed, but entries from different feeds can have the same GUID.

This means that an entry is updated only if an entry is retrieved with the same GUID when fetching the feed to which the entry belongs. If an entry with the same GUID is retrieved when fetching a different feed, it is treated as a different entry and saved independently in the database.
Use current_user instead of user_signed_in? to detect if the user is signed in.

Hopefully this helps with a bug I was experiencing: sometimes the root page was displayed as if the user was not logged in, but when clicking on "sign in" instead of being shown the login form the user was shown the feeds list, with an error message saying that "user was already logged in". This means the code in the root page that detects if the user is logged in was not working correctly.

There are several threads in stack overflow of people complaining about user_signed_in? not working, and some of them report that checking if current_user exists instead gives better results.