public function togglePrivate(Request $request)
 {
     $material = Material::findBySlugOrIdOrFail($request->id);
     if ($material->private == 0) {
         $material->private = 1;
         Session::flash('success', $material->title . ' made private!');
     } else {
         $material->private = 0;
         Session::flash('success', $material->title . ' made public!');
     }
     $material->update();
     return redirect()->back();
 }