[API] Implement totalSeatCount resolver on Account

[spalmurray-codecov]

Implement the new totalSeatCount resolver on the Account object in our GraphQL API. This field should only be accessible to members of one or more Account organizations, but we do not need to worry about that as the Account object is itself only accessible to those users. This value represents the number of seats paid for by the owner, in other words, the sum of the Account model's plan_seat_count and free_seat_count.

See planning doc for more details.