Added REST service for returning Ballot information

* Database schema changes:
    * Ballot:referendumIds is now an array (after talking to Jared)
    * Ballot:candidates:id changed to Ballot:candidates:elementId
    * CustomBallot:ballotResponses:id changed to CustomBallot:ballotResponses:elementId
* Added database relationships for Ballot to:
    * Referenda
    * CustomBallot
    * Candidates
* Added database relationship for CustomBallot to BallotResponse
* Implemented REST service for Ballot