* Convert some CSS to LESS
* Split/Join LESS files
* Remove duplicates between website and web_editor
* UI improvements
* Allow simple customizations thanks to LESS var&mixins for website

The original commit by fka used to define less variables like this:
@px-4: 4px
@px-8: 8px

The goal was to be able to change the margin values in
media queries for smaller screens.
However, as the less is only compiled once in css, those
variables didn't really help as you couldn't redefine them
from inside a media query and have the less magically
reassign the margins on the fly.

I created less helper functions to be able to easily redefine
the margins for smaller screens from inside media queries.

The task also required to replace the nbsp in the html by the
newly created mr and ml classes.

This is also the first phase of the sass -> less transition.