provide API for agenda participants and people who did not vote

[pacokwon]

추가사항:

• Agenda(안건) 모델에 participants 필드 추가 - 해당 안건을 생성할 시 접속해 있던 사람들의 uid 배열로 구성해놓음
• GET /api/agendas에서
  • 현재까지 투표하지 않은 인원 목록 (pplWhoDidNotVote)
  • 안건에 참여한 인원 목록을 전달하도록 수정 (participants)

결론적으로 /api/agendas 의 응답은 다음과 같이 생겼습니다.

[
  {
    _id: '6107ea51ee385f7b67e7a3e8',
    choices: [ '찬성', '반대' ],
    status: 'progress',
    participants: [ 'pacopaco', 'testebea0f' ],
    title: 'asdf',
    content: 'qwer',
    subtitle: 'bs',
    votesCountMap: { '찬성': 0, '반대': 1 },
    createDate: 2021-08-02T12:51:29.407Z,
    expires: 2021-08-02T12:51:38.434Z,
    __v: 0,
    pplWhoDidNotVote: [ 'pacopaco' ],
    userChoice: null
  },
  {
    _id: '6107e253b0b5a36bacf94ad4',
    choices: [ '찬성', '반대' ],
    status: 'progress',
    participants: [ 'pacopaco' ],
    title: 'foo',
    content: 'bar',
    subtitle: 'baz',
    votesCountMap: { '찬성': 1, '반대': 1 },
    createDate: 2021-08-02T12:17:23.044Z,
    expires: 2021-08-02T12:22:08.455Z,
    __v: 0,
    pplWhoDidNotVote: [],
    userChoice: '반대'
  },
  {
    _id: '6107e35cb0b5a36bacf94ad7',
    choices: [ '찬성', '반대' ],
    status: 'progress',
    participants: [ 'pacopaco', 'testebea0f' ],
    title: 'bar',
    content: 'baz',
    subtitle: 'fo',
    votesCountMap: { '찬성': 1, '반대': 0 },
    createDate: 2021-08-02T12:21:48.949Z,
    expires: 2021-08-02T12:22:07.893Z,
    __v: 0,
    pplWhoDidNotVote: [ 'testebea0f' ],
    userChoice: '찬성'
  }
]