调整frontend blueprint目录结构

遇到个问题 [Static files only work for blueprints registered with url_prefix](https://github.com/mitsuhiko/flask/issues/348)

当注册blueprint 未指定**url_prefix**或者指定的**url_prefix**为空时，无法使用blueprint的静态文件

解决办法是给blueprint显式的生命**static_url_path**
```
frontend = Blueprint('frontend', __name__, static_folder='static', static_url_path='/statics')
```