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_