use other mirrors

[abma]

maybe autofetch?

• https://files.balancedannihilation.com/
• http://www.hakora.xyz/files/springrts/maps
• https://zero-k.info/content/maps/
• https://www.springfightclub.com/data/maps/
• auto created .sdz files from rapid builds/

[abma]

not sure how "detect" invalid files: maybe ideally allow clients to report back broken/missing mirrors

[abma]

-> write a simple "crawler" which runs daily?!