Hide LHN if user cannot view pages in the public context

* Move LHN initialization into `CMS.Controllers.LHN`
* Move LHS template code into Mustache templates (template path
  configurable in `templates/layouts/dashboard.haml`
* Hide LHN if user has no `view_object_page` permissions in the public
  context; otherwise it initializes as normal
* Handle `resize_areas` when the resize bar is not needed
* Add `with_model_as` mustache helper to get model information in
  mustache templates