use hbox mixin instead of class

this will allow dynamic css in nbviewer like vbox-ing for low res
without JS or change in html
include hbox in outpuarea css
include vbox into .cell css definition
merge box-flex1 class into .output-area css definition
recompile css
Merge pull request #3174 from Carreau/simple-class

Simple less classes

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 

There are many other places where this can be done.