Send sortIndex with create and update response

[emroussel]

When a user creates or updates any entity on the app (item, kit, custom field, etc.), they get out of order, because the entity in the store is replaced by the one sent back from the api.

Ideally, the api would send the corresponding sortIndex so that entities keep their order.

[AdamVig]

Returning a sortIndex for only the updated or created item would not work, because it’s likely that the sortIndexes of the whole list change any time there is an update. Closing in favor of app/#437.