Add GBIF ID to taxa table

[JimBacon]

David proposes adding the ID from the GBIF Backbone Taxonomy as a new field to our core taxon model to support its increasing use.

To date, some taxon lists have had the external_key field populated with the GBIF ID but we have at least one case (eBMS) where we might want to retrospectively add the GBIF ID to taxa where the external_key is already in use.

[johnvanbreda]

Should we also be considering the ID from the Catalogue of Life? When looking at the 2 taxonomies for the RIS-Ky project, I generally found CoL was easier to search and find a single accepted name - GBIF had quite a few name variants with no records attached.

Another point is that there is an existing table, taxon_codes, which might be able to meet the requirement of storing the GBIF and any other IDs. Though there would need to be enhancements to make this information available in the reporting cache tables and Elasticsearch.