Move invoice update logic onto Invoice#attributes_with_line_items=
Add quiet_assets gem
Update ember-data to support saving records from invalid state

See https://github.com/emberjs/data/pull/1889

rails generate ember:install --channel=canary --ember-data
Display server-side errors when saving invoices

Allow errors on arbitrary properties, not just defined attributes or relationships
See https://github.com/emberjs/data/pull/1984