[v20.05] temp/phosh: hide logout action, add reboot action

Reboot patch is submitted upstream: https://source.puri.sm/Librem5/phosh/-/merge_requests/573

Logout patch is pmos-specific and not upstreamable - there is code to disable (not hide) the button when phosh is not launched by a login manager but even that isn't particularly pretty - the logout doesn't work and would be useless as we have autologin configured in lightdm