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 
The login (or signup screen) after clicking the link looks something like this:

```
You've been invited to view **Mary Smith's** data. Log in to view the invitation.

Username: [john@example.com] (pre-filled, un-editable)
Password: [******]
```

So we need the `email` and the inviter's `fullName`.

Plus, a link like `https://blip.tidepool.io/#/invite?john.example.com` looks less "secure" than something like `https://blip.tidepool.io/#/invite/1242351-skbsdag-235141-dfbhqerqe` no? And you can't easily "expire" the first one if needed...