Organise files for .less compilation, move to 2.3

I'd like to contribute some of my CSS-based 'hacks' that make the Moodle forms look more like Bootstrap forms since it'll be a while before that gets sorted in renderers.

Currently they're mostly in the form of .less files as that lets me re-use the core Boostrap styles more easily.

Since there seemed to be a suprising groundswell of support for LESS I thought I'd lay the groundwork for compiling from .less files.

Since I was copying over .less files from Bootstrap anyway, I copied the v2.3.0 versions.