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
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.