Export blank CSV Fulcrum metadata template

[conorom]

As a way to move towards one canonical source for ingest metadata we'd like to export a blank metadata from the values in the lib/metadata_fields.rb

Part of this ticket is checking that everything is in sync between the latest Google Sheet metadata template in use and the values in the system for import/export.

• verify that the order in lib/metadata_fields.rb matches a recent version of the Google Sheet metadata template
• add field descriptions to lib/metadata_fields.rb using the current second-row values from a recent version of the Google Sheet metadata template. This is going to be a formatting/legibility challenge in the current array-of-hashes setup. Especially as descriptions contain new lines. Do what you can and, if necessary, spin off a second ticket to investigate using a YML file for metadata_fields.rb in future
• verify that all values in the system are matched up correctly with row header values
• tweak or duplicate the exporter code (I think this is the easiest way) to export a blank sheet with just the header and description rows in it
• If the existing exporter code is simply tweaked to export a blank sheet, it seems that the existing exporter spec will suffice
• add a mechanism somewhere in the UI to download a blank sheet (may be separate ticket)
• manually test an import with an exported blank CSV template