Add list_votes_by_voter query to voting_contract

[ueco-jb]

Follow up of: confio/tgrade-contracts#455

We are trying to unify voting contract's interfaces to this pattern:

Rules {},
Proposal { proposal_id: u64 },
ListProposals {
    start_after: Option<u64>,
    limit: Option<u32>,
},
ReverseProposals {
    start_before: Option<u64>,
    limit: Option<u32>,
},
Vote { proposal_id: u64, voter: String },
ListVotes {
    proposal_id: u64,
    start_after: Option<String>,
    limit: Option<u32>,
},
ListVotesByVoter {
    voter: String,
    start_after: Option<String>,
    limit: Option<u32>,
},
Voter { address: String },
ListVoters {
    start_after: Option<String>,
    limit: Option<u32>,
},

[BirgitPohl]

I'm adding this to Patchnet Milestone because this is where we want to have potential breaking changes and not later.
Let me know if this is incorrect.