Refactoring breadcrumbs; adding BreadcrumbHelperBehavior

This makes the breadcrumb navigation clearer, regarding the links to the dashboard, which should appear when coming from either of the user's files or collections views.  This also removes the redundant breadcrumb link to the "Home" page which is duplicated in the navigation header.
The GenericFilesController can access request.referer the same way you can do it in the views.