refs https://github.com/TryGhost/Team/issues/874
In the product benefits modal in Admin, hitting Enter has been updated to work as -
- when hit with focus in an empty benefit field: does nothing
- when hit with focus in any field outside benefit fields: does nothing
- add new benefit and focus on it when hit while on a non-empty benefit
refs https://github.com/TryGhost/Team/issues/792
- updates product benefit to use `name` instead of `label` attribute for benefit text
- updates model/serializer/validator to correctly handle benefit attributes
- added `+` button for adding new labels as the enter behavior is closing the popup(needs fix)
