Mark superseded vocabularies with a link to their replacements

[MattBlissett]

We should consider adding http://purl.org/dc/terms/isReplacedBy to the thesaurus (vocabulary) schema, to allow clearly marking where a vocabulary has been replaced with a newer version — it's not clear from the URL alone.

@mike-podolskiy90, could you check if this would cause any problems with recent releases of the IPT? It's only worth doing if we can update https://rs.gbif.org/vocabulary/gbif/nomenclatural_status.xml and so on, the versions without a date in the filename.

(Via a suggestion received by email.)

Assuming it's OK, adding a general comment field would also be useful.

[mike-podolskiy90]

I don't think adding a new thing to the vocabulary schema would cause any issues in the IPT. I will have a look though.