Add CanAccessPage delegate
Add CanAccessPage delegate

Allows extensions to restrict access to pages.

Notice that if the core logic prevents access to the page, the delegate
is not fired and access is still blocked.

Picked from c36a4b351b

bauhouse/sym-todo-list,michael-e/symphony-2,bauhouse/sym-todo-list,symphonycms/symphony-2,jensscherbl/symphony,michael-e/symphony-2,symphonycms/symphony-2,jensscherbl/symphony