DWCA download verbatim extensions

[MattBlissett]

We now have verbatim extensions available for inclusion in downloads:

https://www.gbif.org/occurrence/download/0000365-240229165702484

https://api.gbif.org/v1/occurrence/download/0000365-240229165702484

The list of available extensions is here: https://api.gbif.org/v1/occurrence/download/describe/dwca which is not all of the known extensions: https://api.gbif.org/v1/enumeration/extension as that includes ones for checklists, deprecated extensions etc.

• Show verbatim extensions on the download page
• Allow selecting verbatim extensions when requesting a DWCA download. Note the verbatim occurrence data is automatically included, as it always has been.