Implement api/file route.
Generate scaffold for downloads.
db/schema.rb: Update.
Make association of download.
Generate scaffold for stored_item.
Make association for stored_item.
db/schema.rb: Update.
config/routes.rb: Update.
app/controllers/api_controller.rb: Implement store.
app/views/stored_items/index.html.erb: Display item name and storage name instead of id.
app/controllers/api_controller.rb: Fix typo.
app/controllers/api_controller.rb: Fix bug in store method.
Display download link string (but not linked) in items#show page.
Link to download item.