Don't offer Fedora distro upgrades if we don't have correct artwork

Make sure we have the artwork for the distro upgrade banner before offering an upgrade. In practice, this means that we don't offer a distro upgrade at all unless the user has first updated gnome-software to the version available from in the updates repo.

This comes with a slight tradeoff that some people who never install regular updates will not get the distro upgrade notification at all after this change.