add two picture and modified the css
      modified:   frontend/themes/default/views/kblog/login.php
      modified:   vendor/yiisoft/yii2-authclient/clients/Tencent.php
      modified:   vendor/yiisoft/yii2-authclient/clients/Weibo.php
      modified:   vendor/yiisoft/yii2-authclient/widgets/assets/authchoice.css
      modified:   vendor/yiisoft/yii2-authclient/widgets/assets/authchoice.js
      new file:   vendor/yiisoft/yii2-authclient/widgets/assets/tencent.png
      new file:   vendor/yiisoft/yii2-authclient/widgets/assets/weibo.png
      new file:   vendor/yiisoft/yii2-authclient_back