GraphQL API for Labels

[sophiedeziel]

Related to #1119

Example queries:

query labels {
  labels {
    edges {
      node {
        id
        name
        color
        files {
          edges {
            node {
              id
            }
          }
        }
      }
    }
  }
}

mutation AttachLabel {
  labelFile(input: {fileId: "27", labelId: "1"}) {
    file {
      id
      labels {
        edges {
          node {
            id
          }
        }
      }
    }
  }
}

mutation CreateLabel {
  createLabel(input: {name: "PLA", color: "#111"}) {
    label {
      id
      name
      color
    }
  }
}

query file {
  file(id: "28") {
    labels {
      edges {
        node {
          id
          name
        }
      }
    }
  }
}