remove more hard coded layout
split notebook style in 2 files,

app specific/not app specific
recompile css
inverse style.less and app.less
min-width instead of width for prompt

mainly helpfull for nbviewer responsive css to have the prompt
above the code cells, and have the correct width.
rename app.less to style_noapp.less
use baseBorderRadius instead of corner_radius

as it already exist in Bootstrap
Merge pull request #4444 from Carreau/css-cleaning

CSS cleaning

- vbox / hbox via classes
- adds notebook/style_noapp.less, which excludes body/toolbar/menubar stuff not necessary in embedded contexts