New automated metadata fields for Git dates (only)

[brockfanning]

Fixes https://github.com/open-sdg/open-sdg/issues/1051

Note: This adds new metadata fields called "national_data_last_updated" and "national_metadata_last_updated", and automatically sets them with the Git date (only). The old fields will continue to be populated, for backwards compatibility. To use the new field and remove the old fields, users will need to change their metadata schema (eg, _prose.yml), like so:

Remove these:

• national_metadata_update_url
• national_metadata_update_url_text
• national_data_update_url
• national_data_update_url_text

Add these:

• national_metadata_updated_date
• national_data_updated_date