Do Authentication in a Plack middleware

[bschmalhofer]

Currently authentication is done in the Kernel/System/Web/Interface* modules. IMHO it would make sense to move authentication into a middleware. This would emphasize that authentication and authorisation is a separate step in processing a request.

[bschmalhofer]

A lighter version of this issue is implemented in #1431. That issue allows support of middlewares for specific interfaces, which was, under the hood, also the main goal of this issue.
Moving the authentication and authorisation logic of index.pl and customer.pl also into a middleware provides no immediate benefits. Thus this issue won't be implemented in the foreseeable future.