Support for adding entries to shopping list without network connectivity

[foo-git]

Possible related to #816.

Use case: I want to add entries to the shopping list at all times, e.g. when having no network connection. You never know under which circumstances you remember you have to buy a product.

Current situation: When phone is in flight mode, press on "+ Add entry" for shopping list, you'll receive an error page explaining that network connectivity is needed to add an entry.

Expectation: Even with flight mode a product should be able to be added to the list or modified. Synchronisation should happen when connectivity is restored. Not sure about conflict resolution, if detection by last-modified-timestamps and subsequent duplication of conflicted entry for later manual resolution is enough.

I like the app, thanks for your time and service for the community.