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 
Thanks for clarifying @cheddar. Just so I make sure I understand correctly, you are suggesting we **completely remove** this endpoint, and have the email contain links that look like:

- `https://blip.tidepool.io/#/login?invitationEmail=bob@example.com`
- `https://blip.tidepool.io/#/signup?invitationEmail=bob@example.com`

The Blip app would then not make any extra HTTP call, just see there is a `invitationEmail` query param, which triggers it to display a message along the lines of "Login/Sign up to view your invitation", and pre-fill the "email" field? 

Is that correct?