Discussion of proposed design for careteam work

Available for review, but please don't merge yet.
In the case this "accept invitation" happens *after* the invitee Mary signed up (for example Mary received an invitation to her email `mary@example.com` and created an account with that email because there wasn't one before).

Does the backend, upon signup, go through all pending invitation for an email *without* an existing account, and associate those invitations with the new account, so when we do:

```
GET /confirm/invitations/456
```

(where `456` is the new userid that was just created with `mary@example.com`)

We will get the invitations previously sent to `mary@example.com` (before the account existed).