Mirror lists

[frauzufall]

Imagine there is a base list (e.g. sites.yml) and a directory called mirrors where people could add mirror servers. One mirror file would contain something like this:

• a list containing IDs of update sites linked to the URLs of their servers
• if they mirrored sites.imagej.net, it would contain the replacement for this path as well
• the location of the mirror server

The updater could then provide the user with a choice to pick from depending on their location.

[ctrueden]

As a starting point, we could have a mirror list for sites.imagej.net. The vast majority of update sites are served from there, and if anyone sets up a mirror (it's got public rsync access), we'd be off to the races. But as this issue describes, we'd still need to augment the updater somehow to check the mirror list, and probably also offer an option somewhere to control which one is used (although we could do automatic detection like fiji.sc's downloads page does).