Jhbate/invite by key

@kentquirk could you take a look please. I still need to do a call,with a server token, to get the inviter user profile but that means a change to go-common 
Ok, I found the trello card where the discussion probably happened: 

https://trello.com/c/YK9Ql93w

However, it is only dealing with email, not with un-authenticated access to the names of people who have sent invitations.  Which might or might not be different.