prevent ../../../ in editor

To prevent ../../../../../ in links and images. 
for ex. adding /images/test.png in "Insert/edit image" dialog will generate:
relative_url: true
```html
<p><img src="../../../../../images/test.png" alt="" /></p>
```
relative_url: false
```html
<p><img src="/images/test.png" alt="" /></p>
```