Restic backups across hosts

[stackptr]

Currently spore uses restic to backup Postgres databases using an environmentFile and passwordFile. This should be extended to make backups on glyph for at least the following:

• ~/.config/beets/library.db
• /var/lib/roon-server/backup

I think hosts/spore/backup.nix should be generalized into modules/nixos/backup.nix so that setting up backups in a host becomes:

rc.backups = {
  roon = {
    paths = ["/var/lib/roon-server/backup"];
  };
};

The module would encapsulate configuration options such as the repository, environmentFile, and passwordFile, which I don't think need to be changed per-host. pruneOpts would have a default that could be overridden in the host.

[stackptr]

Mostly done in #321 -- no need to add extra logic for pruneOpts etc until those need to be overriden