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 
Why not

```
You've been invited to view someone's data.

We apologize that we cannot tell you who, but in order to protect the privacy of the individual who has invited you to view their data, we can only tell you their name once you have logged in and verified ownership of your email address.

Username: [john@example.com] (pre-filled, un-editable)
Password: [******]
```

And

```
https://blip.tidepool.io/#/invite?id=1242351-skbsdag-235141-dfbhqerqe&email=bob@example.com
```

Seems plenty fine to me.