Due to some changes we started notifying via push notifications on other
families of notifications. There are a total of about 30 or so possible
notification you could get, some can be pushed.
This fallback means that if for any reason we are unable to find an icon
for a push notification we just fallback to the Discourse logo.
Also go with a simple reply icon for watching first post.
Note, that in production `image_url` can return an exception if an image is
missing. This is not the case in test / development.
