Add support for mods

[proegssilb]

Add some way for the user to download mods into the server, and keep them automatically updated.

This might be a post-1.0 thing.

[proegssilb]

It looks like some of the larger mods can be 100-200MiB. This means that with 4-5 large mods, the mods can absolutely dwarf the savefile size. That can in turn require resizing a volume if new mods are added, or having to create a volume sized mostly based on the specific mods used, rather than the size of the base or the size of the save file.

On the other hand, most mods are going to be <=10MiB.