Support adding items to empty reference fields

I have added the new setting to the formatter of entity_reference_revisions field type:

When this settings is enabled, frontend editing can display the link to add new items even if field is empty and is not displayed.

This was ported to 2.x branch. Probably will not land in 1.x as 1.x will need to be in feature freeze state.

Automatically closed - issue fixed for 2 weeks with no activity.

a.dmitriiev → closed merge request !67