3rd endpoint: given a filled-in ballot, verify and accept it, returning the ballot check and voter index

[windoverwater]

Preliminary notes:

• if the client passes back a valid filled-in ballot, accept_ballot needs to verify and accept_it.
• would want to add a validate function (perhaps from cast_ballot) if it does not already exist. May need to write one.
• this endpoint returns the ballot check and the voter index id