completeLogin(request: FederationSessionRequest): Promise<User>
Finishes a sign-in begun by startLogin, for
POST /api/federation/sessions, and returns the account it signs in to:
the one linked to the identity, brought up to date, or a new one. A login
works once. A new account needs the home to be trusted, or the invite the
login was started with, claimed now.
request: FederationSessionRequest
Promise<User>