Change host entity of a core

Issue #2966560 on drupal.org by jurgenhaas.

Currently, the entity reference for the host in a core is read-only, once the core entity has been created. Not sure why we've done that but it may well be necessary to change that setting. So let's see if and under which circumstances we can open that up.

Also, when deleting a host entity, the attached cores will be lost without any host assigned. There we should check during deletion, if cores are assigned and if so, ask for a different host before deletion.