Add Board Group Model

[injoonH]

• Add BoardGroup model.

• Add endpoint: GET /board_groups/

  [
    {
      "id": number,
      "slug": string,
      "ko_name": string,
      "en_name": string,
      "boards": [
        {
          "id": number,
          "slug": string,
          "ko_name": string,
          "en_name": string
        }
      ]
    }
  ]