IntegrityError when auto-creating an ImageServer when it already exists as non-validated

1. Create an ImageServer with validated=false.
2. Use CreateIIIFURL or CreateIIIFInformation with an image on that server. You can use an URL that doesn't exist, the error will occur before the check happens. You can use the IIIF import task to trigger those endpoints too.
3. Cry.

ImageServer.objects.from_url tries to create a new server because it filters by validated=True too early; it thinks there are no servers at all when there is one already, just not available.