Add service to create a board list

app/services/boards/lists/create_service.rb

+module Boards
+  module Lists
+    class CreateService
+      def initialize(project, params = {})
+        @board  = project.board
+        @params = params.dup
+      end
+
+      def execute
+        board.lists.create(params.merge(position: position))
+      end
+
+      private
+
+      attr_reader :board, :params
+
+      def position
+        board.lists.size
+      end
+    end
+  end
+end

spec/factories/boards.rb

+FactoryGirl.define do
+  factory :board do
+    project factory: :empty_project
+  end
+end

spec/factories/lists.rb

+FactoryGirl.define do
+  factory :list do
+    board
+    label
+    sequence(:position)
+  end
+end

spec/services/boards/lists/create_service_spec.rb

+require 'spec_helper'
+
+describe Boards::Lists::CreateService, services: true do
+  describe '#execute' do
+    let(:project) { create(:project_with_board) }
+    let(:board)   { project.board }
+    let(:label)   { create(:label, name: 'in-progress') }
+
+    it 'creates a new list for board' do
+      service = described_class.new(project, label_id: label.id)
+
+      expect { service.execute }.to change(board.lists, :count).by(1)
+    end
+
+    it 'inserts the list to the end of lists' do
+      create_list(:list, 2, board: board)
+      service = described_class.new(project, label_id: label.id)
+
+      list = service.execute
+
+      expect(list.position).to eq 2
+    end
+  end
+end