Initialize db on single-user instance config

This fixes the --config step so that when setting up a single-user
instance for the first time (and creating the admin user as part of the
process), the database is automatically initialized before creating that
user.

This removes the need for the --init-db command after --config when
setting up single-user instances.

It fixes #59: "no such table: users" error during the --config step on
single-user instances that haven't previously run --init-db.