regularize schema for external ids

[tgbugs]

The format of external id records is inconsistent. For example see https://api.ror.org/organizations/05rrcem69
/external_ids/GRID/all is a string for a single entry.
/external_ids/Wikidata/all is a list for a single entry.
I think that /external_ids/*/all should always be a list?

Another potential issue is that the ISNI record does not list a preferred even though there is only a single entry. I don't know whether this is an inconsistency or whether ror is reflecting the underlying semantics of the other identifier systems, e.g. ISNI doesn't list a preferred id if there is only a single entry, or may not list a preferred id at all. In either case it is straight forward to implement a simple rule for dealing with null preferred ids so probably not a schema issue.