Scope: write:address-book

Only the fields sent are changed: a field left out is unchanged, and null clears an
optional field. Required fields can't be cleared. address, phoneNumbers and
emailAddresses are replaced whole when sent, as Loop's edit does, and at least one email
address must remain. The agency can't be changed.

user_id is optional: if supplied it must be a Loop agent in the entry's agency, and is
recorded as the agent who changed it; if omitted the agency's first agent is used.