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 
I wrote that based on intuition, but let me try to enumerate reasons why it might be useful

1. I can imagine a flow where, once signed in, we want to highlight the specific invitation that brought the user to the site.  In that case, having the token will provide enough information for blip to make that decision.
1. HTTP access logs generally maintain query parameters and being able to filter by the token could possibly be helpful when trying to make sense of the logs
1. Having the token on the URL provides extra information that could allow Blip to notify a user that the invitation that brought them to the page won't work with a different email address.  That is:
    * Bob gets an invite to address bob@billy.com
    * Bob clicks on the link, recognizes blip and realizes he already has an account at bob@gmail.com
    * Bob goes to sign in instead of sign up, Blip can use the fact that he didn't sign up and the fact that he doesn't have the expected invite associated with him to suggest that Bob adds bob@billy.com to the list of email addresses associated with his account.
1. If someone has an issue with an invite, having the token on the url will help us in figuring out what is going on

That's about all that jumped to the top of my head right now.  I agree that it's not necessary, I was just saying that if I were the only one implementing it, I would include the token on the url with the belief that it doesn't hurt and that I would probably be happy about it in the future.