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.