Add a oneshot systemd service to add Fedora flatpak repos
    
    This adds a systemd service that is type=oneshot and adds fedora and
    fedora-testing (disabled by default) flatpak repos. Various bugs around
    /etc/flatpak/remotes.d/ handling make it difficult to use right now,
    which would otherwise have been a much saner alternative.
    
    See https://github.com/flatpak/flatpak/issues/2508 and
    https://github.com/flatpak/flatpak/issues/1665 for the remotes.d issues.