Add "isFirstYear" property to team members

[cdriehuys]

Users of the API should be able to query for a boolean field called isFirstYear on team members. The boolean should indicate if this is the person's first year as a member of a team.

query IsFirstYear($year: Int!) {
  team(year: $year) {
    members {
      isFirstYear
      person {
        name
      }
    }
  }
}

Acceptance Tests

1. Boolean True for First-Years

If the person associated with a team member is not a member of any prior teams, the field should return true.

2. False for Existing Members

Say we have a person "Joe Smith" who is a member of the 2018 and 2019 teams. His team member record for the 2019 team should return false for isFirstYear.