Give default auth user all permissions, for manual testing

Previously you'd need to modify your local DB to give the first
user the right permissions. This will automatically set up a
custom user with all the required permissions as the default
auth user, that you'll be when running the app locally.

The README auth section can be removed, as it should "Just Work"

Inspired by https://github.com/alphagov/panopticon/pull/219
Good spot, fix PR at https://github.com/alphagov/short-url-manager/pull/13