Function isAddMode

src/components/Msg/MsgForm/index.js:12–12

/* 由于本组件由 /msg/add 与 /msg/:msgId 所公用
   因此需要判断当前是“新增模式”还是“修改模式” */
const isAddMode = pathname => pathname.startsWith('/msg/add')

export default class MsgForm extends Component {
  static contextTypes = {