Customise the auth middleware name

[tobytwigger]

We have an edge-case where we use the auth middleware as a middleware group containing the normal auth middleware, but also email verification and other auth-based middleware. It'd therefore be useful for us to customise the name of the auth middleware in Fortify.

This PR moves the auth name into the Fortify configuration and references it in the routes.php file. It uses auth as the default and so is fully backwards compatible, but gives the end user an option to configure it.