Discussion of proposed design for careteam work

Available for review, but please don't merge yet.
For all these "accept/confirm" (signup, password, invite), all the link in the email needs to contain is the confirmation `type` and `userid`, example:

```
https://blip.tidepool.io/?confirm&type=signup&userid=123
```

I.e., there is not "confirmation key/id" the front-end needs to use in the "accept/confirm" API call? (there seems not, just double-checking)...