Add Boards::Lists::ListService to list lists for a specific board

app/services/boards/lists/list_service.rb

+module Boards
+  module Lists
+    class ListService < Boards::BaseService
+      def execute(board)
+        board.lists
+      end
+    end
+  end
+end

spec/services/boards/lists/list_service_spec.rb

+require 'spec_helper'
+
+describe Boards::Lists::ListService, services: true do
+  describe '#execute' do
+    it "returns board's lists" do
+      project = create(:empty_project)
+      board = create(:board, project: project)
+      label = create(:label, project: project)
+      backlog_list = create(:backlog_list, board: board)
+      list = create(:list, board: board, label: label)
+      done_list = create(:done_list, board: board)
+
+      service = described_class.new(project, double)
+
+      expect(service.execute(board)).to eq [backlog_list, list, done_list]
+    end
+  end
+end