Redirect to home/dashboard page if not adimin user access to /download/.
app/controllers/categories_controller.rb: Add before_action :check_admin.
app/controllers/storages_controller.rb: Add before_action :check_admin.
app/controllers/stored_items_controller.rb: Add before_action :check_admin.
app/views/stored_items/index.html.erb: Display stored date/time.
app/views/items/show.html.erb: Update.