Add more mirrors

I have now sent reminder emails to UNAVCO and NOAA (Walter), plus asked the S. Africa FTP mirror folks if they would be able to serve data as well, and also Dietmar at U Sydney. Checking with @leouieda regarding S America - we have Eder doing the FTP mirror but not sure he has capacity. Once Leo responds I can send an inquiry that way as well. The main weakness today is North America but getting a unavco and noaa would go a long way (US east coast, US Mountain). Perhaps we can get Dave to host a us-west-coast mirror?

_Originally posted by @PaulWessel in https://github.com/GenericMappingTools/gmtserver-admin/issues/18#issuecomment-723482635_

まず、UNAVCO、NOAA、南アフリカ、Sydney、南アメリカのミラー候補について、issue スレッドとリンク先の元の議論を読んでください。提案されたミラーの連絡先と地域カバレッジが解決されれば完了ですが、payload には設定ファイル、テスト、または具体的な実装のエントリーポイントが示されていません。

索引モデルが issue の本文から書いたものです。