AccessDenied page in public store

[RomanovM]

In the admin area, when a user have no access to some resources, we display AccessDenied page. We could add the same in the public store.
Then we could use it when checking permissions, for example in AuthorizeAdminAttribute or CheckAccessPublicStoreAttribute and instead of Challenge (401) result, return Forbid (403) result, and redirect the user to the AccessDenied page.
In this case, we should change "page-not-found" route to a new "access-denied" one here.