Add some kind of financial limits for public use

[markwhiting]

1. Perhaps something on the session level e.g. unregistered users get up to 10 pages of paper
2. Also on the user level e.g. users get 100 pages then some cost model (which we don't need to work out in this issue)
3. Also some holistic max e.g. the platform will stop running (and offering) requests if our account uses more than a fixed limit.