Add existing_plan for invited member leads

In order for Marketing to be able to send more email nurtures to invited members, we need to understand if they are part of a paid namespace. As such we'd like to send existing_plan over with their lead.

Solution to explore

For user registrations that are invites and we send an iterable lead for them(we do similar for pure free registrations) send the existing_plan of the first namespace the user is invited in (note: they can be invited multiple times).