Provide a service that returns archive of Arc contracts

We need to provide a service by which arc-js applications can obtain information about current, prior and future daostack contracts.

Information about future contracts requires a separate external server/source that we may defer doing for now.

For more information, see the prior related issues from the arc repo:
https://github.com/daostack/daostack/issues/138
https://github.com/daostack/daostack/issues/139
https://github.com/daostack/daostack/issues/90

No implementation files or tests are named. Start by reading the linked DAOstack issues and the arc.js_legacy service entry points to determine how current and prior contract information should be sourced; done means applications can retrieve that archive, while future-contract data may remain deferred.

Written by the indexing model from the issue text.