Simple class

Simplify class handeling in js/css

Instead of setting several class (e.g: `cell vbox`) per dom element, use less to embed the definition of `vbox` into `cell`.

This should allow : 
   - more easy theming.
   - responsive css on  nbviewer 

indeed, 
```
In[00] : source
```
Is painfull to read on phones, wherease we could do

```
In[00]
------
source
```
below a certain width.

Also I suppose dealing with less class should make manipulating the DOM faster...

There are many other places where this can be done.
We should be carefull to reflect some changes of this kind into nbconvert.
The one of this PR shouldn't have any visible effects on nbconvert except redondant style in  css.