Create Impact Ballot api

[carlbarrdahl]

Whenever a voter make changes to the ballot it should store the state. A voter can also publish the votes by signing a message.

• Should work similar to OP RF4
• Auto-balance
• Lock metric state (this functionality is useful for ProjectBallot also)
• Ballot votes are currently { projectId, amount } - can we refactor to use { id, amount, locked }?
• Add vote.type in-case both projects and metrics are in the ballot. This helps with querying metrics from ballot.