Create a POSIX user account during first boot

Created by: SunilMohanAdapa

We would like to get rid of default root passwords in FreedomBox. If Plinth were to setup a user account with sudo permissions during the first boot wizard, it should be good enough.

@petterreinholdtsen wrote:

I dropped the well known root password to get rid of at least one well known password on the freedombox test setup. The fbx user have sudo access now instead. In time, when plinth is working well enough for this, I want to get rid of all well known passwords and use the password provided on first configuration in plinth instead.

Initially, we can create this user separately from the Plinth admin user for ease of implementation. Later, we should consider merging Plinth users with POSIX/LDAP users so that it would be possible change user passwords etc. from Plinth. I believe Django will make the implementation easy for us here.