common-workstation: Remove createUser attribute.

No longer needed as of NixOS/nixpkgs@1a75958.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
i3: Fix evaluation of workspaces submodule.

Didn't look into it in detail, but the submodule type no longer seems to
accept module paths directly.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>