When get rich editor value(null) from backend,it will occur error
```
Error in nextTick: "TypeError: Cannot read property 'replace' of null"
```
So I set val to '' to avoid it

detail see https://panjiachen.github.io/vue-element-admin-site/#/zh-cn/lazy-loading?id=%E8%B7%AF%E7%94%B1%E6%87%92%E5%8A%A0%E8%BD%BD

* refactor: Adjust the example directory structure

* perf form demo

* refine editor-slide-upload css

* refine demo

detail see #534

1. 原来的普通导航栏样式会使用 siderbar 的样式，背景黑底。
2. 修改后，sidebar 样式不变，普通导航栏可以正常使用。