Hide collection-type attribute in hibernate mapping files.

A customized collection type had to be added to the hibernate mapping for the class DBStringListProperty for fixing the dirty bit propagation (XWIKI-8398 and XWIKI-8464). But we support users customizing their hibernate mappings, so it is a problem that users may need to update their mapping files.

In order to avoid this, we can instead silently decorate the hibernate configuration with the content-type attribute.