Can't access config module

On a new plinth install, I created a user and logged in. But when I click Configuration, I see this message: "Only members of the expert group are allowed to see and modify the system setup."

Currently there doesn't seem to be any way to add users to the expert group. I think the intention was to remove the expert group functionality? If that's the case, we should remove the expert_mode module, and cleanup any modules/templates/docs that refer to it.