R1:6f3bd13cf5da

Begin adding more guidance to the "One-Time Login" flow

Summary:
Ref T13244. See PHI774. If an install does not use password auth, the "one-time login" flow (via "Welcome" email or "bin/auth recover") is pretty rough. Current behavior:

- If an install uses passwords, the user is prompted to set a password.
- If an install does not use passwords, you're dumped to `/settings/external/` to link an external account. This is pretty sketchy and this UI does not make it clear what users are expected to do…