* add authRequred: 'optional'

* expose auth status via request context

* update security plugin to use notHandled auth outcome

* capabilities service uses optional auth

* update tests

* attach security headers only to unauthorised response

* add isAuthenticated tests for 'optional' auth mode

* security plugin relies on http.auth.isAuthenticated to calc capabilities

* generate docs

* reword test suit names

* update tests

* update test checking isAuth on optional auth path

* address Oleg comments

* add test for auth: try

* fix

* pass isAuthenticted as boolean via context

* remove response header from notHandled

* update docs

* add redirected for auth interceptor

* security plugin uses t.redirected to be compat with auth: optional

* update docs

* require location header in the interface

* address comments #1

* declare isAuthenticated on KibanaRequest

* remove auth.isAuthenticated from scope

* update docs

* remove unnecessary comment

* do not fail on FakrRequest

* small improvements