Quick Setup: add flathub repo with `--user` flag by default

[soredake]

I think it's better to add flathub repo as user repo by default, to avoid potential pollution of /var/tmp (https://github.com/flatpak/flatpak/issues/528 and https://github.com/endlessm/systemd/commit/6bdb355a9e515b7b84ac1f37747603d4db4267c7) on distro's that don't clean up this path (opensuse for example) and to avoid any use of polkit (no entering password's anymore on distro's that require it when working with flatpak). Another reasons for using user over system can be found here https://bugs.kde.org/show_bug.cgi?id=441428#c6