return list of admins as an RPC endpoint

[Varunram]

Admins must be public so that people can contact them in case they face something that they need to seek help for. The admin bool is also returned on querying a user, so this change would not be related to YaleOpenLab/openx-frontend#8. This change should be relatively easy since it involves writing an RPC endpoint and filtering users based on the admin bool.