ProfilePage; Use block_item_blocked in a different way

It used to prevent the callback from being called when we set the
GtkMenuItem's state manually but that's not the case anymore and we can
just use it to prevent people from clicking it multiple times.
ProfilePage: Disable the blocking action

... if the profile's user is the currently logged-in user.
ProfilePage: Add retweet disabling/enabling
Twitter: Catch errors when loading avatars